Evaluating Comparative Research : Mapping and Assessing Current Trends in Built Heritage Studies

The question leading this research is: what are the attributes and scales of comparative research that applies to built heritage studies? The paper begins by recognizing the interrelatedness of built heritage protection and sustainable urban development. While comparative research into built heritage studies analyses and documents existing practices, policies and impacts, its generalizable capacities are often lacking and therefore less applicable to policy-makers. In an attempt to further the potential contribution of such studies, the paper maps comparative built heritage research based on a critical review of over 100 articles and books. The analysis of these sources relies on an evaluative categorization of comparative built heritage studies. This categorization consists of four criteria: the number of compared cases, their geographic location, the scope of comparison and its degree of structuredness. The findings suggest that heritage studies compare a relatively small number of cases; they are quite structured; focus on local as well as national-level analysis; and lean towards Western-centered comparisons. The paper concludes by suggesting that built heritage studies can contribute to sustainable urban development policies by taking on comparative research that has a large enough N, expanding non-Eurocentric and Anglo-American research, comparing local jurisdictions in more than one country and by utilizing highly structured categories for comparison.


Introduction
While in previous years the practical challenges of promoting built heritage protection and sustainable urban development were conceived of as discrete, contemporary conceptualizations and policies view the two as interrelated and mutually-enforcing.For example, the Paris Declaration (2011) of the International Council on Monuments and Sites (ICOMOS) emphasizes the importance of integrating heritage protection as a catalyst for sustainable development [1].In a similar vein, in its call upon governments and policy-makers, UNESCO's Hangzhou Declaration (2013) asserts that "[t]he safeguarding of historic urban and rural areas and of their associated traditional knowledge and practices reduces the environmental footprints of societies, promoting more ecologically sustainable patterns of production and consumption and sustainable urban and architectural design solutions" [2].Likewise, central international resolutions such as the U.N.'s 2030 Agenda for Sustainable Urban Development (2015) urge decision-makers to recognize the role of safeguarding cultural heritage as integral to the goal of making cities "inclusive, safe, resilient and sustainable" [3] (pp. [21][22].The central resolution on sustainable cities and human settlements, the New Urban Agenda asserts that: We will support the leveraging of cultural heritage for sustainable urban development and recognize its role in stimulating participation and responsibility.We will promote innovative and sustainable use of architectural monuments and sites, with the intention of value creation, through respectful restoration and adaptation.We will engage indigenous peoples and local communities in the promotion and dissemination of knowledge of tangible and intangible cultural heritage and protection of traditional expressions and languages, including through the use of new technologies and techniques [4] (p. 22, Article 125).
While linking and integrating the goals of sustainable urban development and heritage protection is highly desirable, this idea did not go unchallenged.To note just a few discrepancies-on the one hand, heritage protection is seen as a catalyst for economic development by reinforcing commerce, tourism and employment, along with increased land values; on the other hand, alteration of existing buildings and monuments may increase land values and local economic viability.With regards to socio-cultural aspects, on the one hand, heritage protection policies support local cultural identities, genius loci and civic memories, thus contributing to community-building, social cohesion, and intergenerational binding; on the other hand, it may also cherish unwarranted historical narratives, undesired social values and generally advance cultural homogeneity at the expense of changing cultural trends, thereby suppressing competing cultures.In environmental terms, it is common wisdom that the most environmentally benign building is the one that need not be built because it already exists, and thus, reduces consumption of new materials in production and avoids waste.On the other hand, old buildings and monuments are frequently built with toxic materials and pollutants, are inefficient in energy and water consumption, and emit a high level of GHG (greenhouse gas).
To address these discrepancies while still meeting the ambitious goals outlined above, various practices, policies and regulations have been employed by national, regional and local governments along with civil society organizations.These practices and policies-and their impact-are constantly studied, documented and analyzed in scholarly research with valuable insights.However, what is frequently lacking is our ability to generalize these conclusions, thus making their applicability in policy-making less likely.Previous research suggests that to fully integrate the goals of heritage protection and sustainable urban development, planners, politicians and other stakeholders may develop strategies and indicators that facilitate the protection of cultural heritage [5].One avenue is to look at best practices, policies and regulations that enable heritage protection and promote sustainable development [6].However, a more robust approach is to examine these policies comparatively [7].To promote the latter, scholarship needs to clearly map the field of comparative research into built heritage, and to set evaluative criteria by which existing research could be analyzed.The contribution of the said evaluative criteria is twofold; first, they enable a comprehensive analysis and mapping of contemporary scholarship; and second, these criteria can be instrumental for future research design.Working towards these goals, the question leading this research is: what are the attributes and scales of comparative research that applies to built heritage studies?Following a preliminary search, 102 publications were shortlisted and critically reviewed; this review enabled us to extract four criteria for assessing the attributes and scales of existing research.These criteria are: (1) number of compared cases, (2) geographic location of compared cases, (3) scope of comparison, (4) degree of structuredness.Comparative publications were mapped accordingly, and the findings offer a comprehensive map of the field.The ensuing discussion proposes several rules of thumb for promoting comparative built heritage research that yields generalizable conclusions, best-practices and policies that enhance the ties between sustainability and built-heritage.These proposals include 'large enough N', expanding non-Eurocentric research, cross-national/cross-local analytic scope and comparison based on highly structured categories.Before addressing those issues, the following section elaborates why it is vital to study built heritage comparatively, in light of sustainability goals.

The Landscape of Comparative Heritage Studies
Scholars in comparative planning and political science often argue that comparison can be a useful tool for cross-national and cross-local transfer of knowledge and can provide a sense of scale to current practices [8,9] (p. 6).Despite scholarly calls to galvanize comparative analysis, single-case studies remain highly prevalent in planning and public policy scholarship.Single-case analysis allows researchers to contextualize their findings, and to learn about relationships between different factors [10]; however, some critics argue that "reliance on a single case does not allow the analyst to assess how robust the findings are . . .cross-sectional survey data, on the other hand, allow for testing relationships across widely divergent settings" [11] (p. 201).Moreover, the case study method, which relies on a single case (or even 'within case') analysis, has been criticized for failing to establish internal and external validity [10] (p.418) and for being too narrow in its scope [12] (p.164).
Unlike single-case analysis, comparative research may be more informative in its analysis of a larger group of cases and items.Therefore, comparison can establish generalizations and help in classifying problems, policies and practices [13] (p. 117), [14] (p.286).It can achieve these goals by employing common frameworks and by establishing criteria for identifying similarities or differences [9,15].It can create classifications that help in understanding how policies and governments work [16]; it can point out shifts in the form of institutional, political, and policy convergence or divergence [17]; and it can be normative and pragmatic in guiding policy-makers [18,19].By doing so, the comparative method is considered as a valid source of information about different contexts which facilitates the introduction and nurturing of sustainable practices, policies, and organizational capacities [20].By learning comparatively what works and what does not, scholars can assist in designing better solutions, adapt existing ones and increase policies' robustness and effectiveness.
Notwithstanding, urban research "has not been very comparative" [21] (p.135).When allegedly 'comparative' work is pursed, comparison is often done in passing, without a systematic approach based on specific criteria for evaluation.Indeed, Kantor and Savitch point out how comparative urban studies rely on 'separate chapters on a limited number of cities capped by an attempt to draw some unifying themes-these works usually turn out to be a compendium of monographs, rather than tightly integrated systematic comparisons" [21] (p.135).A plausible reason for the dearth of studies is that urbanists, planners and policy analysts that focus on the built environment usually adopt a contextual and holistic approach to planning, which is place-based and place-sensitive [22].Comparison may be perceived as a reductionist approach which glosses over local facts, policies and agenda [22].
Moreover, when comparison does happen, certain comparative works have focused on national units and systems, rather than drawing attention to local 'sub-unit' conditions that help in effectively capturing "the intricacies of many social phenomena" [23] (p.800).Thus, pleads for systematic comparative scholarship have been coupled with calls by political scientists to pursue a local view while 'scaling down' the comparative analysis [24][25][26].On the one hand, cross-local studies are advocated, because unlike cross-national investigations, they allow "for greater control of factors such as culture, history, ecology, and socio-economic conditions" [23] (p.803).On the other hand, some critics have warned against a narrow scalar perspective that ignores global processes.As a result, there have been calls to take into account supranational and trans-local elements when conducting comparative work [27].These aspects, localism and global processes, converge in growing instances of autonomous urban policies (i.e., independent of state intervention), therefore intensifying the justifications for comparative cross-local and cross-national research [28][29][30][31].
Within the context of built heritage studies and sustainability, several scholars have pointed out the critical absence of comparative studies that focus on historic preservation and the built heritage.For example, Prudon observed that there is a distinct lack of theoretical knowledge about heritage protection, presumably because of a shortage of comparative work [32].He noted that "nettlesome questions include what and why to preserve.While no specific criteria are given, it is clear that additional comparative analysis will be necessary . . ." [32] (p.X).Hitchcock et al. concurred, noting that no comparative framework exists to compare the effectiveness of heritage management strategies, cross-regionally and internationally [33] (p.266).In addition to the difficulties of urban comparative research noted above, another possible reason for the scarcity of comparative heritage studies is that they face a major hurdle: the great variety of history, geography, socio-cultural and socio-economic makeup makes comparison very challenging [34].
Recognizing the significance of comparative built heritage studies begins by highlighting the importance and epistemological opportunities of comparative research, and the different types, components and features of comparative studies.For example, comparison provides a broad perspective on the motivations, context and socio-cultural aspects of heritage protection, which differs between jurisdictions (both local and national).Through a cross-jurisdictional transfer of knowledge, comparison can help practitioners and policy-makers to devise better policies, to face challenges or to avoid past mistakes [35].Additionally, comparative inquiry is crucial for devising well-informed policies that improve the long-term management of built heritage properties and consequently strengthen the social, economic, environmental and cultural sustainability of those resources [5][6][7]36].Lastly, comparative knowledge is essential in a field of built heritage, which is constantly lampooned as being elitist, random, culturally naïve, unsustainable and even socio-economically destructive [37].Thus, questions such as 'how to preserve', 'why preserve' and 'how to do that in a sustainable manner' become front and center in heritage studies, and can benefit from a comparative outlook.
Putting scholarship aside, countries and localities should not be dealing with the numerous challenges of heritage protection and sustainability in a solitary manner.In this context, comparative research is useful for planners, architects, urban design practitioners and conservationists.By comparing experiences, practices, institutional perspectives and policies in the field of built heritage, the comparative method can 'promote more sustainable management' of cultural properties [38] (p.2).Comparison also relates to socio-political factors, such as civic engagement in planning and conservation, which are often associated with sustainable management of built heritage [38] (p.67).The practical significance of comparative studies is supported by Lowenthal, who posited that comparative work is crucial because it makes our heritage conflicts appear less isolated [39].Specifically, Lowenthal noted that "every legacy is distinctive, to be sure.But realizing our heritage problems are not unique, makes them more bearable, even soluble if we see how time or effort resolved them elsewhere" [39] (p.249).This notion led Murphy to note that comparative research in the field of built heritage is essential.In particular, he argued that "the message is clear: stay alert, proceed with caution, profit from the experience of others in other countries . . ." [40] (p.376).
Over and above this, comparative studies into heritage preservation are supported by the following call: the unmet need is for research that explains how conservation is situated in society-how it is shaped by economic, cultural, and social forces and how, in turn, it shapes society.With this type of research, the field can advance in a positive way by embedding the spheres of conservation within their relevant contexts, informing decision-making processes, fostering links with associated disciplines, and enabling conservation professionals and organizations to respond better in the future . . .[41] (p. 6).
To understand the power of comparative research into built heritage, one may look at a variety of sources e.g., [42][43][44].One of the earliest comparisons is Baldwin Brown's book.Over 100 years ago, Brown established the legacy of comparative work in built heritage studies [42].His seminal book, from 1905, The Care of Ancient Monuments, is a remarkable comparative work that predates modern planning and modern heritage preservation.The book set out the stage for the designation of historic properties to this day.It was also one of the very few that were undertaken in the early 20th century.
Brown's study should be read in its historical context of a growing government concern over built heritage, which led British officials to request Brown's extensive survey [45].Several decades before this comparative work was published, a feverish debate had erupted in Europe about the restoration of aging buildings [45] (p.44).The debate provoked preservationists, some of whom argued for strict restoration or maintenance alone.Brown's comparative work entered this debate with the conviction that buildings are related to the social conditions that produced them and are therefore an integral part of society.Despite its Euro-centric approach, Brown's study signals the ability of comparative studies to inform or transform government and planning altogether.Ever since, comparative studies have been conducted by individual scholars, focus groups, international institutions and advocacy organizations.The comparisons have ranged in scope and subject: some scholars conducted in-depth descriptive studies of institutions and legal frameworks [46]; others engaged in evaluative-comparative research of preservation practices [47].
Summing up, the emphasis on the significance of comparative research on one hand, and the attention drawn to its scarcity on the other, is a notable discrepancy that motivates our inquiry.Although most studies in built heritage focus on a single country or jurisdiction, there is also a body of comparative research that can inform heritage experts.However, we lack a comprehensive account of the major characteristics, strengths and weaknesses of existing comparative research in built heritage.To address this lacuna, we set out to map the field and suggest an evaluative categorization by which we can better assess comparative heritage studies.

Methodology
The analysis was carried out in two stages.In the first stage, we conducted a preliminary review of the literature in the field of built heritage.We surveyed comparative studies that focused on built heritage practices, perceptions and policies.We identified these studies by searching in Web of Science, Google, Google Scholar, ScienceDirect and EBSCOhost search engines-all of which are commonly used by researchers.In addition, we looked for comparative papers and books in websites of heritage organizations and advocacy groups, in scholarly legal databases and the Social Sciences Research Network's (SSRN) website (see Appendix A).
We limited our search to publications from the period 1996-2018.Interestingly, calls by researchers to comparatively examine heritage have been frequently made since the 1990s, e.g., [39][40][41].Thus, the time-frame for our analysis was chosen so as to focus the review on the past two decades; to mirror the state of existing comparative studies into heritage and to examine whether follow-up research had indeed answered these calls.
We excluded essays in the field of engineering that focus on building techniques.Our search did not cover comparative studies in economics and land valuation.Instead, we focused our research on studies that relate to historic buildings and the built heritage, including research that looks at case-examples and implementation issues.
Our literature search covered the field of planning, heritage protection, sustainable management, policy, development studies, tourism and architecture.The general and preliminary search we conducted included several keywords:
Appendix A indicates how many manuscripts were identified in major databases using the chosen keywords.These results were narrowed down using several steps: first, when the output of the search pointed to thousands of results, we applied a narrower search by looking at the Title and/or the Abstract sections using the chosen keywords.Following the search process, for each identified publication, we determined additional relevance by skimming it briefly to validate that it dealt with built heritage.In addition, skimming was designed to verify that the article indeed used the comparative method.Accordingly, we were able to identify a group of publications that focus on built heritage, and out of those, to single out papers with a comparative angle.
After shortlisting a range of publications, we began sorting them.We then thoroughly reviewed each source.Based on this review, we determined four criteria by which we can typify comparative heritage research.In this exploratory framework, our suggested criteria are:

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The 'N' question: How many jurisdictions are compared?

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Geographic scope: What are the locational attributes of comparisons?

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Comparative scope: Is the comparison cross-national (state-level analysis), cross-local (investigating regions, cities, communities) or a combination of those?• Degree of structuredness: Is the comparison structured or unstructured?
These questions were instrumental for mapping the existing landscape of comparative research into built heritage.The data was organized, categorized and coded; the pith of the dataset is provided in Appendix B and fully provided in the website as Supplementary Material.The data was then interpreted and analyzed.The main findings and the analysis are presented in the following section.One limitation we should mention is that the analysis included only articles and books written in English, therefore possibly limiting the ability to fully map the state of comparative heritage studies-especially with regards to geographic locations of comparative heritage studies.However, we are quite confident that the analysis coherently reflects the current state of research and especially the comparative methods employed.In addition, we believe that the suggested categories provide a comprehensive account for evaluation of comparative heritage studies.

Findings & Analysis
We found 102 comparative studies (books, reports and essays) published since 1996.The number of comparative works is impressive; however, it does not imply that the comparative method is frequently applied.Based on our impression of the literature, it appears that comparative studies into built heritage remain the exception, not the rule.With that in mind, we set out to examine the characteristics of existing comparative research.Our findings are presented below, and the discussion is organized according to our suggested evaluative criteria for comparative heritage studies.
Overall, we found that comparative heritage studies examine regulations, policies, management strategies, implementation, threats to and perception of the built heritage, the impact of heritage programs, tourist experience and challenges to policymakers, owners and communities.They also related to socio-economic challenges, and to the dispositions of an array of stakeholders towards the historic environment.Future research may look deeper into these studies while focusing on additional categories of comparison-such as other issues being compared, and the problems being solved.

The 'N' Question
A key issue in comparative studies is the "N Question," which relates to the number of compared jurisdictions.In comparative heritage studies, the number of jurisdictions being compared can vary from two\three, e.g., [48,49] to, in highly ambitious projects, more than ten, e.g., [50].Descriptive statistics of the findings are summarized below in Figure 1.
Figure 1 shows the existence of a large body of comparative research (45%) that focuses on two jurisdictions.For example, Fung et al. compared the legal and organizational system that enables heritage protection in Hong Kong and Macau, finding a vast difference between the two Chinese jurisdictions regarding the scope of protection as well as reuse methods [51].In addition to examining policies and institutional dispositions towards heritage, two-unit analysis compared a range of other issues such as disability access in heritage sites [52], and relocation of heritage structures [53].Two-unit comparisons, also known as binary analysis, enable the comparativist to pinpoint specific aspects and to focus on cases that may present similarity, diversity or variance that would be rendered invisible if a larger sample is compared.While the selection of jurisdictions analyzed may be justified by a variety of logics, it is beyond our scope to analyze these justifications.The selection of cases can be guided by a "most different" research design (i.e., choosing jurisdictions which are highly diverse) or a "most similar" research design based on the premise that systems as identical as possible represent the optimal sample for comparative research.In the event that some important differences are found "then the number of factors attributable to these differences will be sufficiently small to warrant explanation in terms of these differences alone" [54] (pp.[117][118][119][120][121][122][123][124][125][126].At the same time, and in accordance with Mukhija's critique, comparing a small number of jurisdictions (whether specific sites, cities or states) does not necessarily maximize the explanatory powers of comparative research [10].
The analysis shows that only one third of the studies undertook a comparison of five jurisdictions or more, while the vast majority compared fewer cases.Notably, sample size (number of cases) chosen for comparative research depended on a range of considerations, including the complexity of the subject matter at hand, resource availability (time, language familiarity), available data and the extent of the comparative task undertaken by the researcher.The fact that few studies in the field of built heritage employed a multi-jurisdiction analysis of more than five cases may indicate on one hand that existing scholarship risks being too narrow in its comparative scope.However, on the other hand, "with too broad a range of cases, the differences become overwhelming and the common object of inquiry vanishes" [55] (p.2).Despite this caveat in comparative heritage studies, "evidence from multiple cases is often considered more compelling" [56] (p.131).Thus, we propose that comparative heritage studies should move forward to include larger samples which present a more robust analysis.Indeed, the data accentuates the need to compare a larger set of jurisdictions or cases.

The geography of comparative heritage studies
The analysis identified a large group (46.5%) of comparative research that focused on European and/or Anglo-American jurisdictions (see Figure 2).For example, an extensive comparative work by Pickard brought together a compilation of informed essays by professionals from 13 European countries [44].The study may be criticized for its Eurocentricity, however, the aim of this scholarly work was, notably, to guide European member states and to provide grounds for possible unification or shared European understanding of heritage protection.Pickard's work illustrates the wide- Two-unit comparisons, also known as binary analysis, enable the comparativist to pinpoint specific aspects and to focus on cases that may present similarity, diversity or variance that would be rendered invisible if a larger sample is compared.While the selection of jurisdictions analyzed may be justified by a variety of logics, it is beyond our scope to analyze these justifications.The selection of cases can be guided by a "most different" research design (i.e., choosing jurisdictions which are highly diverse) or a "most similar" research design based on the premise that systems as identical as possible represent the optimal sample for comparative research.In the event that some important differences are found "then the number of factors attributable to these differences will be sufficiently small to warrant explanation in terms of these differences alone" [54] (pp.[117][118][119][120][121][122][123][124][125][126].At the same time, and in accordance with Mukhija's critique, comparing a small number of jurisdictions (whether specific sites, cities or states) does not necessarily maximize the explanatory powers of comparative research [10].
The analysis shows that only one third of the studies undertook a comparison of five jurisdictions or more, while the vast majority compared fewer cases.Notably, sample size (number of cases) chosen for comparative research depended on a range of considerations, including the complexity of the subject matter at hand, resource availability (time, language familiarity), available data and the extent of the comparative task undertaken by the researcher.The fact that few studies in the field of built heritage employed a multi-jurisdiction analysis of more than five cases may indicate on one hand that existing scholarship risks being too narrow in its comparative scope.However, on the other hand, "with too broad a range of cases, the differences become overwhelming and the common object of inquiry vanishes" [55] (p.2).Despite this caveat in comparative heritage studies, "evidence from multiple cases is often considered more compelling" [56] (p.131).Thus, we propose that comparative heritage studies should move forward to include larger samples which present a more robust analysis.Indeed, the data accentuates the need to compare a larger set of jurisdictions or cases.

The Geography of Comparative Heritage Studies
The analysis identified a large group (46.5%) of comparative research that focused on European and/or Anglo-American jurisdictions (see Figure 2).For example, an extensive comparative work by Pickard brought together a compilation of informed essays by professionals from 13 European countries [44].The study may be criticized for its Eurocentricity, however, the aim of this scholarly work was, notably, to guide European member states and to provide grounds for possible unification or shared European understanding of heritage protection.Pickard's work illustrates the wide-ranging subjects covered by built heritage research: exploring comparatively a variety of definitions of "heritage" in several countries and examining a range of other issues such as the identification procedure of heritage, its protection, funding, the role of specialist organizations and preservation education.related issues, some of which include the Global South.
It is possible that our mapping was somewhat constrained because it was limited to Englishlanguage publications.Built heritage research at the local scale (comparative and non-comparative alike) is often published in the local language owing to its focus on ongoing domestic dilemmas and challenges.Therefore, it is reasonable to assume that more research exists, not necessarily Eurocentric or in Anglo-American localities.Given this limitation, we do not suggest that our analysis fully depicts the state of comparative research in built heritage.Consequently, future research expanding the analysis would be most welcome.However, we are still quite confident that the data are sufficiently indicative given the fact that English is the most common language for academic research and for international policy transfers.After identifying the regions being compared and the locational attributes of comparative heritage research (see Figure 2), we set out to examine the cross-border aspect of comparison.A good way to describe this is by mapping the trajectories of comparative scholarship, namely locating points and routes of comparison.Figure 3 provides an infographic description of our findings.These comparisons are important in enabling the consolidation of laws or even cross-European inspiration and transfer of knowledge.However, they also confirm critiques about the Euro-tinted analyses in planning studies.As Chakrabarty has argued, the time is ripe to 'provincialize Europe' [57].Following suit, Robinson argued we should look beyond North America and Europe when studying urban phenomena and challenges [58].Indeed, the findings (Figure 2) also point to a growing interest in comparative heritage studies that focused on the Far East and Asia.Additionally, there is a relatively large body of research that applied cross-continent examination of heritage-related issues, some of which include the Global South.
It is possible that our mapping was somewhat constrained because it was limited to English-language publications.Built heritage research at the local scale (comparative and non-comparative alike) is often published in the local language owing to its focus on ongoing domestic dilemmas and challenges.Therefore, it is reasonable to assume that more research exists, not necessarily Euro-centric or in Anglo-American localities.Given this limitation, we do not suggest that our analysis fully depicts the state of comparative research in built heritage.Consequently, future research expanding the analysis would be most welcome.However, we are still quite confident that the data are sufficiently indicative given the fact that English is the most common language for academic research and for international policy transfers.
After identifying the regions being compared and the locational attributes of comparative heritage research (see Figure 2), we set out to examine the cross-border aspect of comparison.A good way to describe this is by mapping the trajectories of comparative scholarship, namely locating points and routes of comparison.Figure 3 provides an infographic description of our findings.What emerged from the graphic rendering was the impressive geographic coverage of comparative studies.And yet, it appeared that cross-border comparisons are primarily concentrated in three major 'poles', namely Europe, North America and -to a certain extent-the Far East, while Latin America, Africa, the Middle East, Asia and Australia were less represented, at least from a comparative perspective.Additionally, the infographics illuminated another aspect-the existence of a large body of cross-national analyses, alongside local comparisons.We discuss this issue in the following section.

The Comparative Scope of Built Heritage Studies
Another aspect that emerged from the data relates to the geographic scope of the comparative analysis.In this regard, we found three major types of studies: cross-national analyses, cross-local analyses in a single state and cross-local analysis that is also cross-national.Cross-national comparisons focused mostly on national-level planning, national-level policies, regulations, practices, impact and the histories of heritage protection.The unit of analysis in cross-national research was a country or an entire region in the world.Within federative, non-unitary countries, the unit of analysis may be a state within the federation.
Cross-local comparisons, on the other hand, included micro-level analysis of smaller units (cities, villages, specific sites, etc.).As a whole, cross-local comparisons focused on the locale and might compare municipalities' capacity, local communities' dispositions and grassroots activity relating to built heritage.Some of these cross-local studies focus on cases in one country while others were both cross-local and cross-national (i.e., comparing local cases in several countries and hereafter called 'cross-local/cross-national').The distinction between the two makes a difference in terms of the comparative scope and the type of yielded conclusions.
This classification (Figure 4) indicates that 39% of the sources employ a cross-national comparison, 29% are cross-local and another 29% are cross-local/cross-national.Overall, the findings show that comparative built heritage studies mostly employ a local lens and constitute 59% of the identified publications.It is, therefore, quite evident that the local foci is dominant in comparative heritage studies in relation to those that conduct cross-national comparisons.In the following, we discuss and characterize each of these classifications.
Sustainability 2019, 11 FOR PEER REVIEW 10 What emerged from the graphic rendering was the impressive geographic coverage of comparative studies.And yet, it appeared that cross-border comparisons are primarily concentrated in three major 'poles', namely Europe, North America and -to a certain extent-the Far East, while Latin America, Africa, the Middle East, Asia and Australia were less represented, at least from a comparative perspective.Additionally, the infographics illuminated another aspect-the existence of a large body of cross-national analyses, alongside local comparisons.We discuss this issue in the following section.

The comparative scope of built heritage studies
Another aspect that emerged from the data relates to the geographic scope of the comparative analysis.In this regard, we found three major types of studies: cross-national analyses, cross-local analyses in a single state and cross-local analysis that is also cross-national.Cross-national comparisons focused mostly on national-level planning, national-level policies, regulations, practices, impact and the histories of heritage protection.The unit of analysis in cross-national research was a country or an entire region in the world.Within federative, non-unitary countries, the unit of analysis may be a state within the federation.
Cross-local comparisons, on the other hand, included micro-level analysis of smaller units (cities, villages, specific sites, etc.).As a whole, cross-local comparisons focused on the locale and might compare municipalities' capacity, local communities' dispositions and grassroots activity relating to built heritage.Some of these cross-local studies focus on cases in one country while others were both cross-local and cross-national (i.e., comparing local cases in several countries and hereafter called 'cross-local/cross-national').The distinction between the two makes a difference in terms of the comparative scope and the type of yielded conclusions.
This classification (Figure 4) indicates that 39% of the sources employ a cross-national comparison, 29% are cross-local and another 29% are cross-local/cross-national.Overall, the findings show that comparative built heritage studies mostly employ a local lens and constitute 59% of the identified publications.It is, therefore, quite evident that the local foci is dominant in comparative heritage studies in relation to those that conduct cross-national comparisons.In the following, we discuss and characterize each of these classifications.The smaller proportion of cross-national studies may be explained by the challenges associated with cross-border analysis, which is quite difficult to conduct.Vexing questions and dissimilarities between countries are abundant, originating in different historic, geographic and socio-political contexts.This led Slater to recommend that in analyzing heritage policies and managerial The smaller proportion of cross-national studies may be explained by the challenges associated with cross-border analysis, which is quite difficult to conduct.Vexing questions and dissimilarities between countries are abundant, originating in different historic, geographic and socio-political contexts.This led Slater to recommend that in analyzing heritage policies and managerial frameworks one should be aware of the politics, histories and geographies of difference [59].Despite such difficulties, cross-national (and country-wide) comparisons of built heritage practices can contribute immensely to the advancement of the field.There are many examples of excellent comparisons of this type.For example, Irsheid focused on regulatory mechanisms and policies in different Arab countries in order to explore their experiences and best practices [60].Likewise, other scholars related to an extensive corpus of laws, guidelines, the flexibility of current administrative framework, decision-making processes and statutory and non-statutory instruments [49,61].However, the cross-national inquiries did not attempt an in-depth review of local, municipal-level practices.Despite this limitation, cross-national comparisons are useful for deciphering local circumstances because the cross-border scope provides a yardstick for comparing cities, neighborhoods, localities, local planning authorities, communities or even single historic buildings and their relationship with national institutions, norms and regulations.
Alongside national-level comparisons, the data highlights the dominance of sub-national analysis, which scaled down the comparison.Interestingly, we found that contrary to other fields of urban inquiry, built heritage research does look at the local sphere comparatively.This runs contrary to Kantor and Savitch's critique of urban scholarship, according to which "many scholars who undertake comparative studies of nations do not venture into cities" [21] (p.136).
Sybblis and Centeno argue that 'scaling-down' the comparative-prism, i.e., examining local sub-national practices, is of essence, because "the global move toward decentralization and the seeming retreat of social groups to their distinct corners, imply that subunits (both states and local governments), sectors and regions may have more explanatory value in our current world than previously" [23] (p.800).
This observation stands out when taken into the field of built heritage where historic protection is often exercised at the local level [62] (p.92) [37].The local dimension of heritage protection is quite strong: older structures manifest themselves locally in brick and stone; local values are embedded in heritage [63]; local distinctiveness has become a major factor for preservationists [64]; built heritage as a form of cultural heritage is tightly related to politics of identity [65] (p.80) [66] (p.88) [67] and politics of identity "are more clearly and convincingly present in institutional and policy responses at the local levels of government" [68] (p.20).Moreover, heritage conflicts are also primarily associated with statutory protection which is often carved out by city administration [69] (p.2415).Thus, comparative local studies into built heritage are of essence.These studies can become important in light of calls to empower local authorities and communities [70].Furthermore, local comparisons can answer "a growing sense of localism, channeling a need for local identification in an increasingly transnationalized world" [71] (p.97).
Overall, the local prism in comparative heritage research focused on what happens in the "local field" of inquiry.This implies that the inquiry touches on many local issues, for instance: How do tourists react to heritage?How do planning officers balance local needs?How do they achieve sustainable development while protecting heritage?How are local stakeholders involved in heritage protection?Examining questions of this type comparatively proved to be useful for understanding historic places, hence, our assertion that local practices and knowledge are important for sustaining built heritage.Studies that compare local problems, impacts, standpoints and conflicts can be as inspiring as those that examine national-level heritage planning.Moreover, national-level inquiry alone provides at best a starting point for comparison, whereas cross-local research grounds national-level studies in local practices [72] (p.20).
The focus on the local brings to the fore controversial issues that are difficult to pinpoint while examining the national scale, such as local character, aesthetics and politics.However, cross-local research is not without its detractors.Cross-local comparisons are "not straightforward, because of the enormous task and long list of criteria that could be used in the comparison, and because of the institutional complexity and important differences between different local governments within [sic.] countries" [73] (p.13) [74] (pp.2-3).
In this regard, we distinguished between cross-local comparisons within the same jurisdiction and cross-local/cross-national comparisons.To exemplify the former, Lovelady's study compared local models for built heritage protection, and its cross-local approach highlights the importance of flexible preservation mechanisms [75].Lovelady's focus was on neighborhood conservation districts (NCD) in the USA and on how different models of districts are created and handled.Lovelady asserted that NCDs offer less stringent regulatory hurdles and greater flexibility in managing the historic environment.He argued that NCDs are part of a pluralist tradition of conservation in the US and that they promote contextual practices.Lovelady's comparison introduced and compared two types of NCDs and discussed their efficacy in protecting historic districts.Additional comparative studies exemplified how local problems can help address general heritage dilemmas faced globally [76][77][78].These studies were therefore imperative for gaining invaluable insight into shared problems.
The selection of single-state practices avoids the complexity of matching and choosing several countries for comparison.In addition, comparison within a single state simplifies the analytical endeavor in that it enables the examination of several institutional systems (localities) that operate within the same national-level framework.This means that the researcher can utilize a "most-similar design" for the research.Local examination can also contribute to a larger sample size (owing to issues of information accessibility), which can raise the explanatory power of the study.
At the same time, a single-state comparison is limited in its scope and may not be as useful for policymakers who wish to gain a broader perspective on policy design or management.Therefore, cross-local/cross-national should be harnessed to push forward the bandwagon of heritage studies.Cross-local research that compares local issues in different countries opens the hatch for bringing together experiences that operate in a variety of contexts.
A few examples of this type of comparisons clarify our point.Mullin et al. compared the preservation of four waterfront cities in the USA and Portugal [79].Their analysis pointed out great similarities between preservation strategies in the two countries.The authors stated that "although American cities differ substantially from their European counterparts, the . . .case studies from both sides of the Atlantic Ocean illustrate . . .[a] planning paradigm that places historic preservation at the center of all waterfront planning strategy" [79] (p.47).They concluded that "at the local level, integration of historic preservation ordinances in planning documents, volunteer co-operations with historical societies and civic organizations, and educational campaigns can get good results" [79] (p.55).This comparison led the authors to offer some 'best practice' guidelines for protecting heritage.These recommendations would not have been possible had this study focused solely on the national scale.
City-specific cross-national heritage research was also present in Roth's essay [80].Similarly to Mullin et al. [79], Roth compared four cities around the globe: New York, Berlin, Cairo and Tokyo.The research is interesting not only owing to its cross-local scope but because it also selected a controversial issue for comparison: examining how cities protect their recent (20th century) past.Roth compared enabling legislation in each city and identified the key agencies responsible for the nomination of historic properties.Interestingly, New York City, the youngest of the four cities, was characterized as being at the forefront of heritage policy with its highly professional preservation commission and its strenuous effort to keep in touch with its vernacular environment.Roth's cross-local research signaled an important shift from national to local heritage policies devised, interpreted and implemented by city governments.
Last but not least, Tweed and Sutherland conducted cross-local/cross-national research comparing Copenhagen, Liege and Belfast [81].In doing so, the authors tied sustainable urban development to cultural heritage.More pointedly, they compared the attitudes of city dwellers towards the historic urban environment and found that in all cities, interviewees demonstrated conservative attitudes towards change, but differed in terms of their awareness to interventions in the historic urbanscape.
Clearly, the abovementioned studies are but a drop in an ocean of cross-local research.The common feature in these studies is that they accept the premise that "planning contexts vary not only amongst different nations . . .but also within nations" [47] (p.250).As a result, cross-local studies provide a fuller and more robust account of contextual matters, bringing to the fore nuance and case-sensitive analysis.

Degree of Structuredness in Comparative Heritage Studies
The last criterion for typifying comparative built heritage research is based on the degree of structuredness of comparisons based on the classifications and systematic analysis they employ [21] (p.136).Accordingly, we defined a structured analysis as one that employs a clear framework, criteria or thematic categorization; unstructured research brings various cases into the inquiry, though the comparison is less systematic; semi-structured research does employ some form of categorization, though it does not run through systematically.
To elaborate on these suggested categories, an unstructured comparison sheds light on many important findings; however, these observations were often scattered, or mentioned in passing without using a systematic framework for comparison.Under this category, the research briefly glossed over comparable elements and often discussed them briefly as concluding remarks.Alternatively, in the 'unstructured' genre, cases were discussed separately, without much deliberation on comparative issues.Structured comparisons, on the other hand, highlighted similarities and differences more clearly; they thematically organized the comparison around key issues or concepts, while clearly discussing the comparable elements, weaving together different observations from case-studies and conducting a rigorous integrative analysis.In structured analysis, the comparison runs steadily throughout the study and similarities and differences may be accentuated through the use of a table or figures that highlight comparable variables [21].
While coding the literature, we observed a third category, which we termed 'semi-structured'.Semi-structured sources compared cases, but the comparison was only partially structured; it may be very brief, at times anecdotal or appear as a short analysis that summarized the findings.In semi-structured research, comparison was limited in scope and did not run throughout the analysis.Instead, comparison often followed case-studies discussed separately in an insular manner, or-alternatively-described and lumped the cases together without clearly identifying precise comparable elements.
Utilizing these categories (Figure 5), we found that 63% of the sources employed a highly structured framework to compare built heritage preservation; 23% of the comparisons were unstructured, and 14% were semi-structured.This implies that the majority of comparative built heritage research was based on systematic comparisons that accentuated similarities and differences by applying an integrative analysis.The integration enables heritage scholars, policymakers and preservationists to grasp and organize a set of complex policies and data in the field of built heritage, and to place the compared jurisdictions within context.For example, De Rosa and Di Palma took it upon themselves to suggest key objectives that should direct future policies for the sustainable development of Naples along with preservation of its built heritage [82].Accordingly, De Rosa and Di Palma compared Naples with three other European cities that underwent similar projects.The compared cities shared a common affinity regarding the relationship of the cities' historic centers to their waterfronts.The comparison was structured around an original framework of sustainability indicators and focused on best policy recommendations.Drawing on this cross-local/cross-national inquiry, the authors put themselves in a highly capable position to argue that Naples should align its historic center with the sea and should reconsider existing urban regeneration plans.Following a thorough comparison, the authors offered best practices taken from other comparable cities.
very brief, at times anecdotal or appear as a short analysis that summarized the findings.In semistructured research, comparison was limited in scope and did not run throughout the analysis.Instead, comparison often followed case-studies discussed separately in an insular manner, oralternatively-described and lumped the cases together without clearly identifying precise comparable elements.Utilizing these categories (Figure 5), we found that 63% of the sources employed a highly structured framework to compare built heritage preservation; 23% of the comparisons were unstructured, and 14% were semi-structured.This implies that the majority of comparative built heritage research was based on systematic comparisons that accentuated similarities and differences by applying an integrative analysis.The integration enables heritage scholars, policymakers and preservationists to grasp and organize a set of complex policies and data in the field of built heritage, and to place the compared jurisdictions within context.For example, De Rosa and Di Palma took it upon themselves to suggest key objectives that should direct future policies for the sustainable development of Naples along with preservation of its built heritage [82].Accordingly, De Rosa and Unstructured studies, on the other hand, did not integrate the findings, leaving this task to the reader, e.g., [49].This form of comparative research is sometimes called "juxtapositional analysis" [47] (p.5).However, unstructured comparative research is not entirely juxtapositional, because according to comparativists, juxtaposition should begin with a description of the rules and then clearly indicate similarities and differences by contrasting and comparing data [83] (p.59).However, unstructured studies leave the reader with the task of inferring evaluative-comparative conclusions.To some extent, this runs contrary to methodological rules of comparative policy research, according to which the comparativist "must proceed to a critical evaluation of what he has discovered ( . . . ) In fact the comparativist is in the best position to follow his comparative research with a critical evaluation.If he does not, no one else will do it" [84] (pp.[46][47].This categorization does not imply that unstructured comparative research is not valuable.Unstructured comparisons collect and document plenty of data from several jurisdictions; in so, they are valuable for flagging common challenges and/or solutions in the realm of built heritage and sustainability.

Conclusions: Comparative Research Concerning Built Heritage Preservation
The four evaluative criteria we used in the analysis provide an overview of the state of comparative research into built heritage.Together, these categories form a set of criteria that function as an evaluative standard for assessing comparative built heritage studies.However, we should stress that each of these categories is independent and that there is no necessary theoretical and/or empirical connection between them.Each category has its benefits and challenges for comparative analysis, which are summarized in Table 1 below.Overall, these criteria may be instrumental for characterizing existing research and for designing future comparative studies.Runs the risk of not paying sufficient attention to small details and nuances; thus, socio-cultural context and meanings of built heritage might be overlooked.

Unstructured
Collecting data about several jurisdictions and themes; their value is in developing, putting forward and flagging issues and/or challenges associated with built heritage.
Loosely comparative and do not necessarily provide an integrated analysis of policy, nor overarching observations about built heritage.

Semi-structured
Focuses on specific comparable elements, while avoiding a rigorous and comprehensive comparative analysis.
Relatively structured, but does not provide a thorough comparison that runs throughout the analysis.
Based on these suggested criteria, our normative conclusion regarding built heritage preservation and sustainable urban development goes beyond illustrating the importance of comparative heritage studies.The foregoing analysis indicates that there is a growing need to help built-heritage studies advance sustainable development goals, by looking at heritage comparatively.While this article scanned the landscape of comparative heritage studies without delving into the particularities of each comparison, there is much to be learned in the future by analyzing the content and findings of the studies surveyed.Specifically, future research can explore the conclusions that stem out of these comparisons in terms of specific policies, practices, outcomes and challenges.The data we collected also suggests that there is a growing need to integrate a broader set of cases in a cross-national\cross-local scope; that there is an urgent need to expand the geographic scope of comparison beyond Euro-centered and Anglo-American research; and that scholars should look at heritage in a more structured manner while conducting comparisons.In addition, regarding comparative heritage studies, we suggest that further analysis extends its scope by examining publications in other languages and by assessing their contribution in protecting built heritage.
Although these conclusions stem directly from our analysis of comparative heritage studies, the suggested evaluative criteria can also inform and guide comparative studies in planning, urban politics and governance studies.Put differently, comparative research that addresses the built environment, its politics and its management is fraught with similar challenges, not to say maladies, of cross-jurisdictional analysis.Hence, by re-examining their scope and orientation, using the abovementioned criteria, it becomes possible to put comparative studies back on the agenda.We also searched comparative heritage studies in the websites the following heritage organizations and advocacy groups:

Figure 1 .
Figure 1.Descriptive statistics of the number of compared cases.

Figure 1 .
Figure 1.Descriptive statistics of the number of compared cases.

Figure 2 .
Figure 2. Frequency of locational attributes of comparative studies into built heritage.

Figure 2 .
Figure 2. Frequency of locational attributes of comparative studies into built heritage.

Figure 3 .
Figure 3. Geographic setting of comparative built heritage studies.

Figure 3 .
Figure 3. Geographic setting of comparative built heritage studies.

Table 1 .
Summary and evaluation: comparative built heritage studies.