LEED-EB Gold Projects for O ﬃ ce Spaces in Large Buildings Transitioning from Version 3 (v3) to 4 (v4): Similarities and Di ﬀ erences between Finland and Spain

: This study aims to assess the similarities and di ﬀ erences between Finland and Spain in terms of Leadership in Energy and Environmental Design for Existing Buildings (LEED-EB) Gold large o ﬃ ce building-type projects transitioning from version 3 (v3) to version 4 (v4). The percentages of the average scores are used here to assess the achievements of the LEED-EB data. The natural logarithm of the odds ratio ( ln θ ) and Fisher (cid:48) s exact 2 × 2 tests with a mid p -value are used to evaluate dichotomous data, while the exact Wilcoxon–Mann–Whitney and Cli ﬀ (cid:48) s δ e ﬀ ect size tests are used to evaluate ordinary data. The results for LEED-EB Gold large o ﬃ ces demonstrate similar certiﬁcation strategies in Finland and Spain. These results may be useful to LEED-EB practitioners in Finland and Spain for facilitating the selection of appropriate certiﬁcation strategies in line with identiﬁed high-performance credits for large o ﬃ ces.


Introduction
In 1998, the United States Green Building Council (USGBC) initiated the Leadership in Energy and Environmental Design (LEED) as the U.S. rating system for building sustainability certification, which has been continually improved from version to version [1]. LEED can currently be applied to various building types, such as LEED for New Construction and Major Renovations (LEED-NC), Commercial Interiors (LEED-CI), Core and Shell Development (LEED-C-and-S), and Existing Buildings Operations and Maintenance (LEED-EB), among others. These systems evaluate the following categories: location and transportation (LT), sustainable sites (SS), water efficiency (WE), energy and atmosphere (EA), materials and resources (MR), indoor environmental quality (EQ), innovation and design (ID), and regional priority (RP). Each of these categories contains anywhere from one to several credits. These credits prescribe the relevant requirements and award different numbers of points, according to the importance of a given issue in building-related sustainability. As a result, buildings can be certified as follows: Certified (40-49 points), Silver (50-59 points), Gold (60-79 points), or Platinum (80 points and above).
LEED is also well-known as an international system, as it is in demand in many countries around the world [2]. In Europe, LEED is popular in such countries as Italy, Turkey, Spain, Finland, and Sweden [3]. At the same time, LEED has often been criticized for its non-adaptability to foreign countries, as they have different climates, demographics, technologies, and cultural features than the U.S., for which LEED was initially created [4]. As a result, LEED introduced regional priority points and alternative compliance paths for East Asia, Europe, and South America [5].
With the flexibility of the LEED system, each project can choose an appropriate certification strategy based on its social, environmental, and economic goals. This process is a challenge, due to the by improving insulation and HVAC effectiveness results in less materials being required and less waste generation [17]. However, retrofitting also presents many risks and uncertainties related to the selected retrofitting techniques and approaches, restricted financial and schedule frames, and the significant energy saving payback period [18].
The literature describes several cases of successful implementations of LEED certification in building retrofits. For example, Dall'O et al. analyzed the option of improving school sustainability in northern Italy through the application of LEED-NC v3 requirements [19]. Sun et al. studied the Chow Yei Ching (CYC) University building in Hong Kong which, in 2015, received LEED-EB v3 Gold certification after the application of energy efficient retrofit measures [20]. Mazzola et al. used LEED-EB version 4 (v4) certification to suggest green retrofit measures for a museum (Ca' Rezzonico) in Italy [21]. Thus, green certification is useful, as it prescribes directions and tools toward the sustainable retrofitting of existing buildings [22].
In addition, all the aforementioned LEED foreign-related studies analyzed only the projects certified under the previous version (v3) of LEED certification [5,[9][10][11]13,14]. However, the current version (v4) of LEED [23] certification differs in several respects from its predecessor [24]. This is the case for all LEED systems, including LEED-EB. Thus, in LEED-EB, the SS category of version 3 was reformulated into the LT and SS categories in version 4, and some credits were reformulated and re-evaluated (in terms of their maximum possible points) in version 4, compared with their evaluation in version 3 (e.g., the EQc5 Daylight and Quality Views credit has four maximum awarded points, compared with the EQc2.4 Daylight and Views credit in version 3, which has only one maximum awarded point). Thus, there is a different distribution of maximum possible points in LEED v3 and v4 categories. In particular, LEED v3 includes SS (26 points), WE (14 points), EA (35 points), MR (10 points), and EQ (15 points) categories [24], whereas LEED v4 includes LT (15 points), SS (10 points), WE (12 points), EA (38 points), MR (8 points), and EQ (17 points) categories [23]. Thus, for more precise evidence for changing the certification strategies in LEED foreign-based countries, versions 3 and 4 need to be evaluated in parallel. However, no such empirical research-that is, research which analyzes the same building type of LEED-EB v3 and v4 projects in Europe-has yet been conducted.
Thus, the present study aims to fill this gap by studying European LEED-EB v3 and v4 certifications for large office buildings in Finland (northern Europe) and Spain (southern Europe).
The main research questions are the following. Are there similarities or differences in the certification strategies of the same building type (i.e., large offices) in countries with similar, European-based community values and different natural energy and water resources, as well as different climate conditions? Have these similarities or differences changed when improving from LEED-EB version 3 to version 4? It is worth noting that only LEED-EB v3 and v4 projects at the Gold level have a sufficient sample size to statistically assess differences between Finland and Spain, so only Gold projects are considered here. Figure 1 shows the three sequential steps for data collection and two types of data analysis. Data collection and analysis procedures are presented in Sections 2.2 and 2.3.

Data Collection
The achieved credits of LEED-EB v3 and v4 Gold projects were collected from the USGBC office site [25], while space types and project sizes were collected from the Green Building Information Gateway (GBIG) global platform [26]. The USGBC and GBIG databases discovered 14 LEED v4 Gold large office-type (>6435 m 2 ) projects in Finland and 16 in Spain. Furthermore, the following properties were recorded: the number of projects, the city, and the last certification time [25,26]. Based on the USGBC and GBIG databases, we collected the same number of LEED-EB v3 large office-type (>6435 m 2 ) projects with the same properties, as identified in LEED-EB v4 [25,26]. This design structure was used to reduce the influence of uncontrollable factors when two independent groups were to be compared [27,28]. Table 1 demonstrates that, for both versions, the differences between building sizes certified in Finland and Spain seemed to be negative. Table 2 shows that versions 3 and 4 of the LEED-EB Gold projects showed a similar distribution among cities in Finland and Spain. The p-values were evaluated according to the following three-valued logic: seems to be positive (bold font); seems to be negative (ordinary font size); and judgment is suspended (italic font).

Data Analysis
The percentage of average score (PAS)-the ratio of achieved points to maximum points (expressed as a percentage)-was used to assess the performance of LEED data [29]. If the LEED data had a dichotomous structure, then the natural logarithm of the odds ratio (ln θ) [30] and Fisher s exact 2 × 2 tests were used [31]. If the LEED data had an ordinal structure, then Cliff s δ effect size [32] and exact Wilcoxon-Mann-Whitney (WMW) [33] tests were used. These non-parametric tests were used as the assumption of normality was not met [34].

Effect Size Interpretation
If ln θ was zero, then there was no association between the LEED points and Finland or Spain. The left and right limits of ln θ were infinite (i.e., the function could change infinitely in either direction away from zero) [35]. Consequently, a value of ln θ moving away from zero reflected a stronger relationship between the LEED points and Finland or Spain.
The non-parametric Cliff's δ test was applied to measure the magnitude of the difference of the two distributions (i.e., the effect size) [30]. Cliff's δ ranged between −1 and +1; positive (+) values indicated that the Finland LEED data was larger than the Spain LEED data, zero indicated an equality or overlap, and negative (−) values indicated that the Spain LEED data was larger than the Finland LEED data [30]. Table 3 shows the absolute effect size thresholds (small, medium, and large) for |ln θ| and Cliff's δ. The three levels for effect size thresholds had the following definitions. "A medium effect is visible to the naked eye of a careful observer. A small effect is noticeably smaller than medium but not so small as to be trivial. A large effect is the same distance above the medium as small is below it" [38]. It is worth noting that the effect size did not constitute "iron-clad criteria" [39], but was only a general rule of thumb that may be followed in the absence of knowledge of the area [40].

p-Value Interpretation
A Neo-Fisherian significance assessment (NFSA) was used instead of the paleo-Fisherian and Neyman-Pearson paradigms (i.e., null hypothesis significance tests) [41]. The NFSA paradigm includes the following definitions: (1) the paradigm does not fix α (i.e., the level of significance); (2) the paradigm does not describe p-values as significant or nonsignificant; (3) the paradigm does not accept null hypotheses based on high p-values, but only suspends judgment; (4) the paradigm presents effect size information in conjunction with p-values; and finally, (5) the NFSA paradigm interprets p-values based on three-valued logic; namely, either they seem to be positive (i.e., there seems to be a difference between Finland and Spain), seem to be negative (i.e., there does not seem to be a difference between Finland and Spain), or judgment is suspended (regarding the difference between Finland and Spain) [41].
Hurlbert and Lombardi cited Fischer s philosophical proposal that "no scientific worker has a fixed level of significance at which from year to year, and in all circumstances, he rejects [null] hypotheses; he rather gives his mind to each particular case in light of his evidence and ideas." [41] (p. 316). In addition, Beninger et al. noted that the logic of Occam s razor should not be used for universal interpretation of the p-value [42]. Therefore, the neo-Fisherian paradigm was used to interpret the signs and magnitudes of the statistical effects [41]. Table 4 shows the results for the SS and LT credits and categories. According to these results, in Spain, SSc4 (Alternative Commuting Transportation in version 3) and LTc1 (Alternative Transportation in version 4) had high PAS values (80 and 89, respectively). In contrast, Finland had a low PAS (44) in version 3 of the certification. However, in version 4 of the certification, Finland improved its achieved score a lot (PAS = 72).  Table A1. a Fisher's exact 2 × 2 test and the ln θ test were used. b The Wilcoxon-Mann-Whitney (WMW) exact test and Cliff s δ test were used. p-values were evaluated according to the following three-valued logic: seems to be positive (bold font); seems to be negative (ordinary font size); and judgment is suspended (italic font).

Sustainable Sites (SS) and Location and Transportation (LT) Credits and Categories
For SSc7.1 (Heat Island Reduction-Non-roof in version 3) and SSc3 (Heat Island Reduction in version 4), Spain had a higher PAS than Finland, with 100 versus 79 in SSc7.1 (version 3) and 53 versus 36 in SSc3 (version 4). Spain also showed better results than Finland for the two additional credits of version 3 of the certification, namely SSc2 (Building Exterior and Hardscape Management Plan) and SSc3 (Integrated Pest Management, Erosion Control, and Landscape Management Plan (LMP)).
In terms of Light Pollution Reduction (SSc8 in version 3 and SSc4 in version 4), Finland's PASs increased from 36 to 79, and Spain's PASs increased from 13 to 75. However, for SSc6 (Stormwater Quantity Control in version 3), the PASs were 7 and 13 for Finland and Spain, respectively, while for SSc2 (Rainwater Management in version 4), the PASs were zero for both countries.
Thus, with regards to version 3 of the certification, Finland and Spain mostly had different achievements for credits, with only one similarly high credits achieved and three similarly low credits achieved (Table 3). However, considering version 4 of the certification, less than half of the credits were differently achieved, with increasing numbers of similarly high credits achieved and decreasing numbers of similarly low credits achieved. For almost all of these differently achieved credits (in both versions of the certification), Spain performed better than Finland. Thus, in the SS category (in versions 3 and 4), Spain presented better achievements than Finland.   Table A2. a Fisher's exact 2 × 2 test and the ln θ test were used. b The Wilcoxon-Mann-Whitney (WMW) exact test and Cliff s δ test were used. p-values were evaluated according to the following three-valued logic: seems to be positive (bold font); seems to be negative (ordinary font size); and judgment is suspended (italic font).

Water Efficiency (WE) Credits and Category
In contrast, for WEc1 (Water Performance Measurement in version 3) and WEc3 (Water Metering in version 4), the version-related improvement tendency was reversed. Namely, the PASs of Finland and Spain decreased between version 3 (50 and 81, respectively) to version 4 (11 and 25, respectively). Overall (see Table 4), three differently achieved credits and one similarly low credit were revealed for Finland's and Spain's LEED-EB v3 credits. As a result, in this version, Finland outperformed Spain in the WE category. However, in LEED-EB v4, WE credit achievement was improved, with two similarly high credit achievements in both Finland and Spain. Thus, in this version, both Finland and Spain had similarly high achievements in the WE category. Table 6 shows the results for the EA credits and categories. EAc4 (On-Site and Off-Site Renewable Energy in version 3) and EAc6 (Renewable Energy in version 4) were of low priority for both countries (PAS = 25-27 for both countries), with the only exception being Spain's PAS for the LEED-EB v3 certification (48). In contrast, the issue of operational energy saving was of high priority for both countries. This was correct for both versions of the certification, showing PASs of 92 (Finland) and 87 (Spain) in version 3 (EAc1, Optimize Energy Efficiency Performance) and 91 (Finland) and 85 (Spain) in version 4 (EAc8, Optimize Energy Performance).  Such EA credit achievements resulted in similar patterns for their distributions in three groups of achievements in versions 3 and 4 of the certifications (Table 5). In particular, most credits were achieved similarly in both Finland and Spain, with almost equal numbers of credits achieved similarly high as to those achieved similarly low. Table 7 Table 8 shows the results for the EQ credits and categories. Only one credit, EQc2.4 (Daylight and Views), of the 15 total EQ credits was achieved to a similarly high degree (PAS = 64 and PAS = 63 in Finland and Spain, respectively) in version 3 of the certification. Interestingly, in version 3, for EQc2.1 (Occupant Comfort-Occupant Survey) and EQc3.2 (Green Cleaning-Custodial Effectiveness Assessment), Finland showed better results than Spain (PAS = 71 and PAS = 79 for Finland and PAS = 38 and PAS = 44 for Spain, respectively). However, in EQc1.1 (Indoor Air Quality Management Program) and EQc3.6 (Green Cleaning-Indoor Integrated Pest Management) in version 3, Spain showed better results than Finland (PAS = 69 and PAS = 63 for Spain and PAS = 21 and PAS = 21 for Finland, respectively). In version 4, Spain increased their achievement in EQc10 (Occupant Comfort Survey) and EQc6 (Green Cleaning-Custodial Effectiveness Assessment), while Finland increased their achievement in EQc1 (Indoor Air Quality Management Program) and EQc9 (Integrated Pest Management). In this way, in version 4, these four credits resulted in similarly high achievements for both Finland and Spain (PAS = 63-100). In addition, EQc7 (Green Cleaning-Products and Materials) also performed well in Finland and Spain, with high PASs of 71 and 52, respectively. Thus, in version 4, 5 of the 10 total credits had high PASs (52-100 for both countries).

Indoor Environmental Quality (EQ) Credits and Category
EQc2.3 (Occupant Comfort-Thermal Comfort Monitoring in version 3) and EQc3 (Thermal Comfort in version 4) showed low PASs (0-7) in both countries. EQc1.2 (Indoor Air Quality Best Management Practices-Outdoor Air Delivery Monitoring) and EQc3.1 (Green Cleaning-High-Performance Cleaning Program) in version 3, as well as EQc4 (Interior Lighting) in version 4, also received low PASs (0-43) in both Finland and Spain.
Finally, in version 3 of the certification, Finland showed better results than Spain in 7 of the 10 total differently achieved credits. As a result, in the EQ category, Finland had a higher PAS than Spain (50 versus 37). However, in version 4 of the certification, Spain showed better results than Finland in two of the three differently achieved credits and, in seven credits, Finland and Spain had similar achievements. Therefore, both countries had very similar PASs of 48 and 49 in Finland and Spain, respectively (see Table 8).

Sustainable Sites and Location and Transportation Credits
Public transportation is an issue that can be easily solved in highly urbanized countries through highly developed public transportation infrastructure [4]. Most of the analyzed projects were located in high-density cites such as Helsinki (Finland) or Madrid and Barcelona (Spain), as shown in Table 2. Thus, the high performance of both Finland and Spain, in terms of public transportation issues (SSc4 in version 3 and LTc1in version 4, Table 3), was expected.
The urban heat island (UHI) reduction issue (SSc7.1 in version 3 and SSc3 in version 3 and version 4, Table 3) was more popular in Spain than in Finland. This is because this issue has higher relevance in Spain's climate, which includes five weather groups: oceanic, continental, Mediterranean, semi-arid, and sub-tropical [43]. Such climates mainly have hot summers with high solar radiation. In particular, most projects considered in this study were located in Madrid (sub-tropical climate) and Barcelona (Mediterranean climate). In such climates, the UHI effect can reach 4.3 • C at street level, increasing cooling demands and thus building energy consumption [44].
For the next two issues-light pollution reduction and rainwater management-similar approaches were found for Finland and Spain. The light pollution reduction issue (SSc8 in version 3 and SSc4 in version 4, Table 3) was improved in both Finland and Spain when changing from version 3 to version 4.
However, the rainwater management issue (SSc6 in version 3 and SSc2 in version 4, Table 3) was found to be unattractive in both countries. Among the five aforementioned weather groups of Spain, only oceanic climates are characterized by high rainfall, whereas the other four climate groups (continental, Mediterranean, semi-arid, and sub-tropical) are characterized by limited or scarce rainfall [43]. Thus, the rainwater management issue is not exactly relevant to Spain. In contrast, temperatures and rainfall differ significantly in southern and northern Finland [45]. However, most of the projects considered in this study were located in Helsinki (humid continental climate) and Espoo (cold and temperate climate), both of which feature significant rainfall throughout the year. Thus, the rainwater management issue seems to be more relevant to Finland. However, as was noticed, this credit was given little to no attention in this country.

Water Efficiency Credits
Compared to version 3, in version 4 of the certification, the indoor (WEc4 in version 4) and outdoor (WEc1 in version 4) water saving issues were improved in both Finland and Spain (Table 4). This was despite the differences in the country-specific currents and potential water resources. Finland (northern Europe) has adequate sources for water supplies [46], whereas Spain (southern Europe) experiences high water stress [47]. It is expected that, in future decades, in northern Europe, precipitation patterns may change due to climate changes (e.g., more precipitation in winter and less precipitation in summer) [48], whereas in southern Europe, cumulative annual precipitation may decrease, especially in Mediterranean areas [47]. However, in terms of the water metering issue (WEc1 in version 3 and WEc3 in version 4), performance decreased from version 3 to version 4 (Table 4).
Finally, the cooling tower water saving issue (WEc4 in version 3 and WEc2 in version 4) performed poorly in both Finland and Spain (Table 4). Cooling tower water-related credits require the reduction of potable water for cooling towers and evaporative condensers [23,24]. Such heating, ventilation, and air conditioning (HVAC) systems are usually installed in large buildings [12] (p. 17). Thus, the low popularity of this issue in the large offices of both Finland and Spain seems surprising.

Energy and Atmosphere Credits
The renewable energy issue (EAc4 in version 3 and EAc6 in version 4) had low performance in both Finland and Spain (Table 5). This was surprising, as the share of renewable energy constitutes approximately one third of all produced energy in both countries. In Finland, renewable energy is produced by hydropower (28%) and wind, solar, and biofuel (3%) processes [49], whereas in Spain, it is produced by wind (19%), solar (5%), and biomass (11%) processes [50].
In contrast, the issue of operational energy saving (EAc1 in version 3 and EAc8 in version 4) was of high priority in both countries (Table 5). Thus, large offices in Finland and Spain emphasized the operational energy saving issue. The operational energy saving issue depends on the decisions of building design teams to improve the building envelope (e.g., wall and roof-related insulation and thermal mass and window-related solar and thermal insulation) and HVAC efficiency [23,24]. In European countries, decreasing the operational energy of buildings has been strongly motivated by European Union-related development strategies for moving toward a low-carbon economy by 2050 [51]. In this respect, Finland has targeted the reduction of district heating fossil fuel-based emissions by 80% by 2050 [52]. In Spain, the basic energy efficiency document (energy conservation in the Spanish building code) restricts thermal conductivity (the U-value), among other parameters, to reduce operational energy in the building sector [53]. Therefore, it was not surprising that the issue of operational energy saving was of high priority in both countries for both versions of the certifications.

Materials and Resources Credits
In both versions of the certification, only the sustainable purchasing of lamps with reduced mercury (MRc4 and MR3 in versions 3 and 4, respectively) and solid waste management (MRc6 and MRc1 in versions 3 and 4, respectively) were popular issues in both Finland and Spain (Table 6). All other sustainable purchasing, with regards to ongoing consumables, electric-powered equipment, furniture, facility alterations, and ongoing and facility maintenance (MRc1, MRc2.1, MRc2.2, and MRc3 in version 3 and MRc4 and MRc5 in version 4) were considered to be unattractive issues by both Finland and Spain (Table 6). These sustainable purchasing criteria require purchasing goods that contain certain percentages of post-consumer, post-industrial, rapidly renewable, and salvageable components from on-site materials [23]. The replacement of virgin materials after manufacturing has always been an unattractive and difficult issue [54].
The MR category has also shown low achievement in other LEED systems and countries: LEED-NC v3 projects certified around the world; LEED-CI v3 and LEED-C and S projects certified in Turkey, Spain, and Italy; LEED-NC v3 projects certified in the U.S., China, Turkey, and Brazil; and LEED-CI v4 projects certified in the U.S. [2,5,14,55]. In all these LEED systems, MR credits have different formulations, depending on the type of certified building, such as newly constructed buildings, existing buildings, commercial interiors, or core and shell. Regardless, this category is generally not considered popular among building practitioners. This issue should attract the attention of LEED experts, in terms of encouraging the attractiveness of these credits in building communities.

Indoor Environmental Quality Credits
Thermal comfort issues (EQc2.3 in version 3 and EQc3 in version 4) were considered unpopular in both Finland and Spain (Table 7). This credit requires installing systems for tracking thermal comfort by measuring air temperature and humidity in occupied spaces [23,24]. It seems that this credit should be important in large offices, due to the high density of occupied spaces. Thus, it was unexpected to see low interest in this issue in both countries.
The three additional issues of outdoor air delivery monitoring (EQc1.2 in version 3), a highperformance cleaning program (EQc3.1 in version 3), and interior lighting (EQc4 in version 4) were also unpopular in both Finland and Spain ( Table 7). The outdoor air delivery monitoring issue involves the installation of monitoring systems to measure CO 2 concentrations in occupied spaces. The high-performance cleaning program issue requires using sustainable cleaning materials with decreased contents of hazardous chemicals. Lastly, the interior lighting issue encourages the provision of high-quality lighting by installing individual lighting controls for building occupants [23,24]. These credits also seem to be important for densely occupied offices. Thus, it is interesting that both Finland and Spain paid little attention to these credits. Table 9 shows the high-frequency credits that were very helpful for achieving LEED-EB v4 Gold certification in office-type buildings located in Finland and Spain.

Conclusions
The transition from LEED version 3 to version 4 has provided more flexible rating systems, such that each country can use its own certification strategy at a local level. However, these strategies may not be unique to all building types. In this study, LEED-EB v3 and v4 Gold large office-type (>6435 m 2 ) projects in Finland and Spain-countries with similar values in the European community, but different resources and climates-were analyzed.
The following trends were noticed in the shift from version 3 to version 4 LEED-EB Gold projects. In the LT, SS, WE, and EQ categories, the number of differently achieved credits between Finland and Spain decreased, whereas the number of similarly high credits of achievement in these countries increased. However, in the EA and MR categories, the number of similarly high and similarly low credits achieved between Finland and Spain remained almost the same. As a result, in version 4, the high-frequency credits in Finland and Spain for LEED-EB v4 Gold certification were almost the same (Table 9).
This suggests that the certification strategies for large offices in Finland and Spain were more similar than different. These results may be useful for LEED practitioners in Finland and Spain in order to facilitate the selection of appropriate strategies for LEED-EB v4 large offices.
In addition, it can be concluded that, when improving from LEED-EB version 3 to version 4, each of these countries had possibilities to improve their performance. Finland showed improvement in the transportation issue (LT), and Spain showed improvement in outdoor water reduction (WE) and indoor air quality management issues (EQ). This can be seen as empirical evidence that LEED-EB has become more flexible in the transition from version 3 to version 4.
This study fills the gap associated with a lack of empirical research in LEED certification design, with a specific data collection framework aimed at reducing the impact of uncontrollable factors, such as different certification levels, different building types, and different building sizes. Thus, the certification strategies disclosed here can be considered as more robust than those recommended by previous studies, which did not account for such uncontrollable factors.
A limitation of this study is the current small number of LEED-EB v4 projects in Europe. In order to better develop certification strategies, more research is required, ideally taking into account the future accumulation of LEED projects in foreign countries.
Funding: This research received no external funding.

Conflicts of Interest:
The author declares that there is no conflict of interest.