The Effects of Non-Tariff Measures on Agricultural Trade Efficiency of South Africa Within the SADC

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In the paper titled " The Effects of Non-Tariff Measures on Agricultural Trade Efficiency of South Africa within SADC". I present the following critiques.

First, please abbreviate SADC in the abstract. While the paper highlights the importance of distinguishing between natural and man-made trade barriers, it does not discuss existing theoretical frameworks that explain how non-tariff measures (NTMs) specifically affect trade efficiency. I recommend enhancing the literature review to include the mechanisms through which NTMs obstruct trade, such as political economy issues, regulatory complexity, and institutional capacities.

The choice of a stochastic frontier gravity model is suitable; however, the details regarding model specification, robustness checks, and variable selection are lacking. Discuss potential measurement errors and conduct sensitivity analyses to bolster validity. Moreover, consider incorporating more robust methods to evaluate the impacts of both non-tariff and tariff measures. The descriptive analysis cannot support claims about the impacts or causality of the independent variables, as outlined in Table 2. The authors should reanalyze the data using alternative methods, such as GMM or Driscoll-Kraay, etc. I suggest reviewing a relevant paper that utilized the stochastic frontier model for guidance (https://www.sciencedirect.com/science/article/pii/S0959652622000609).

Although the paper advocates for harmonization, it provides limited insight into practical challenges such as sovereignty issues, capacity constraints, and political resistance. I recommend adding discussions on how these obstacles could be overcome. For the literature review, please include recent regional studies, WTO/UNECA reports, and contemporary empirical research on trade facilitation to strengthen your analysis. Please add future research directions.

Authors should address the following questions:

1. Which specific non-tariff measures contribute most to trade inefficiency?
2. How can SADC countries collaborate to harmonize NTMs and enhance trade flow? Is there proof that some countries or policies are better at reducing negative impacts?
3. How are non-tariff measures quantified in your analysis, and what limitations exist in this measurement approach?
4. How does your empirical model account for potential endogeneity between NTMs and trade flows, ensuring that observed relationships reflect causal impacts rather than reverse causality or omitted variables?
5. What practical challenges and political considerations arise from harmonizing NTMs across SADC member states, and how do your findings on agricultural trade efficiency apply to other sectors or regions within SADC, impacting regional integration strategies?

 

Author Response

Dear Editor and Reviewers

Thank you for the opportunity to revise my manuscript. I am grateful for the valuable feedback provided by you and the reviewers. I have carefully considered each of the comments and have made substantial revisions to the manuscript in response.

Below, I provide a detailed response to the comments raised by the reviewers:

Reviewer 1.

1. First, please abbreviate SADC in the abstract.

I have done so, thank you.

2. While the paper highlights the importance of distinguishing between natural and man-made trade barriers, it does not discuss existing theoretical frameworks that explain how non-tariff measures (NTMs) specifically affect trade efficiency. I recommend enhancing the literature review to include the mechanisms through which NTMs obstruct trade, such as political economy issues, regulatory complexity, and institutional capacities.

Thank you for raising this. I have addressed the comments by adding a theoretical framework and empirical literature. The mechanisms are also explained in the theoretical framework. These changes are highlighted in yellow for ease of reference.

3. The choice of a stochastic frontier gravity model is suitable; however, the details regarding model specification, robustness checks, and variable selection are lacking. Discuss potential measurement errors and conduct sensitivity analyses to bolster validity. Moreover, consider incorporating more robust methods to evaluate the impacts of both non-tariff and tariff measures. The descriptive analysis cannot support claims about the impacts or causality of the independent variables, as outlined in Table 2. The authors should reanalyze the data using alternative methods, such as GMM or Driscoll-Kraay, etc. I suggest reviewing a relevant paper that utilized the stochastic frontier model for guidance (https://www.sciencedirect.com/science/article/pii/S0959652622000609).

Thank you for raising this. I have addressed the comment by adding a theoretical framework which provides a theoretical basis from which the empirical specification is derived. I have also performed a robustness exercise using an alternative estimator that is robust to potential endogeneity. Diagnostic tests have also been performed and reported to support the models used. The proposed GMM or Discroll Kraay required that I estimate the stochastic gravity model using a two-step approach which, as I stated in the methodology section is subject to a bias (see the references below). The one I applied in this study as a robustness check (Karakaplan, 2022) is consistent with the one-step approach recommended in literature and it addresses endogeneity using instruments similar to the GMM approach. I also reviewed the article you recommended. I felt, however, that the paper overly cites equations that may inadvertently confuse a non-technical reader without adding much value.

Wang, H.J. and Schmidt, P., 2002. One-step and two-step estimation of the effects of exogenous variables on technical efficiency levels. journal of Productivity Analysis, 18, pp.129-144.

Karakaplan, M.U., 2022. Panel stochastic frontier models with endogeneity. The Stata Journal, 22(3), pp.643-663.

I agree with you that descriptive analysis cannot support claims about the impacts or causality of the independent variables. You may have, however, missed the clarity provided in the original submission in the first paragraph of section 3 of the initial submission and section 4 of the revised. The table actually summarizes observation-specific marginal effects. To make this clearer, I have added a section of marginal effects in the methodology section which provides the clarity that readers may need in their understanding of Table 2.

This section presents the estimation results from the stochastic frontier gravity model, which decomposes total agricultural trade between South Africa and its SADC trading partners into potential trade (the frontier) and trade inefficiency. The model is estimated using panel data, where South Africa is the exporting country, and 11 SADC member states are the importers. The dependent variable is the total bilateral agricultural trade value. Robust standard errors are used to guide against potential heteroscedasticity. The results are presented in two tables. Table 1 presents the estimated baseline regression containing the frontier model and the inefficiency specification. Table 2 presents the unconditional marginal effects of tariffs and NTMs on trade inefficiency since the coefficients in Table 1 are conditional on the stochastic error term.

 

The added section in the methodology is as follows.

 

3.2 Marginal Effects

In stochastic gravity frontier model of international trade, the relationship between tariff and NTMs and the inefficiency term is inherently non-linear due to the composite error structure which comprises both the random noise and the inefficiency component. As a result, the estimated coefficients in the inefficiency equation do not directly indicate the marginal effects of the covariates on inefficiency. Instead, the marginal effects must be computed to account for the interaction between each covariate and the distributional assumptions of the inefficiency term. This is particularly important since the conditional inefficiency term is derived from a non-linear function involving probability density and cumulative distribution functions. Against this background, we computed the marginal effects post estimation following the derivation proposed by Wang (2002).

where  is a vector of variables affecting trade efficiency (i.e., costs arising from tariff and NTMs),  and  and  are the corresponding coefficients. Wang (2002) demonstrates that the marginal effect of a  variable on trade inefficiency is nonmonotonic implying that the impact depends on the values of exogenous and may vary across observations. It is therefore necessary to compute observation-specific marginal effects and summarize to observe the average impact of tariff and NTMs.

 

4. Although the paper advocates for harmonization, it provides limited insight into practical challenges such as sovereignty issues, capacity constraints, and political resistance. I recommend adding discussions on how these obstacles could be overcome. For the literature review, please include recent regional studies, WTO/UNECA reports, and contemporary empirical research on trade facilitation to strengthen your analysis. Please add future research directions.

Thank you for these comments and suggestions. I have enhanced the discussion section to accommodate this. As you will notice in yellow sections, I have particularly added a discussion around sovereign issues and how such challenges may be overcome. I have also added contemporary literature including a WTO report in the empirical literature review which similarly found a relatively large negative effect of NTMs on trade. I have also added a limitations section which includes areas for further research.

Authors should address the following questions:

1. Which specific non-tariff measures contribute most to trade inefficiency?

I have provided a clearer explanation of how this study proxied NTMs. This is found under data description in the methodology section, and it is highlighted in yellow. I have also added, in the limitations section, a paragraph that explains how, despite the strength of using trade costs data, the study is unable to separate the effects of different types of NTMs. I am of the view that the benefit of using trade costs arising from NTMs (as a proxy variable) outweighs the above limitation to the extent that they provide a way of directly gauging the burden borne by exporters.

1. How can SADC countries collaborate to harmonize NTMs and enhance trade flow? Is there proof that some countries or policies are better at reducing negative impacts?

Thank you for raising an important practical question. I have added some paragraphs in the discussion addressing this in line with your recommendation.

1. How are non-tariff measures quantified in your analysis, and what limitations exist in this measurement approach?

This is explained in the methodology section as indicated below.

In the inefficiency equation, the study includes two variables capturing trade costs arising from tariffs and NTMs. Rather than focusing on specific NTMs such as sanitary and phytosanitary measures, technical barriers to trade, or import/export licenses, the study employs aggregate trade cost data derived from the Economic and Social Commission for Asia and the Pacific (ESCAP)-World Bank trade cost database. This approach offers several advantages. First, using trade costs as a variable captures the overall economic impact of NTMs rather than using specific measures. While notifications of NTBs or SPS measures indicate the presence of regulatory restrictions for instance, they do not provide direct information on the cost implications of these barriers for traders. In contrast, trade costs data, measured in ad valorem terms, quantify the actual economic burden imposed by NTMs, thus allowing for a better assessment of their impact on trade flows (Chen & Novy, 2012). In addition, this methodology facilitates a direct comparison of the relative costs associated with tariffs and NTMs (Anderson & van Wincoop, 2004).

I have included the limitations under the limitations section.

Second, the study employs aggregate NTM trade costs data without distinguishing between different types of NTMs (e.g., sanitary and phytosanitary measures, technical barriers to trade, import/export licenses). As a result, the study is unable to provide specific policy recommendations targeted at particular NTM categories that may have distinct effects on trade efficiency. Future studies could consider disaggregating NTMs to better capture their heterogenous effects.

 

1. How does your empirical model account for potential endogeneity between NTMs and trade flows, ensuring that observed relationships reflect causal impacts rather than reverse causality or omitted variables?

Thank you for the comment. I have addressed potential endogeneity as alluded to earlier using the instrumental variable stochastic frontier model proposed by Karakaplan (2022). The results of this robustness exercise are presented in Table 3. This addition is highlighted in yellow in the results section.

1. What practical challenges and political considerations arise from harmonizing NTMs across SADC member states, and how do your findings on agricultural trade efficiency apply to other sectors or regions within SADC, impacting regional integration strategies?

Thank you for raising this. Kindly see the discussion section which has been enhanced to address this in line with your suggestions which I agree with. Regarding the applicability of the results to other SADC, I have cited this as a limitation (see below).

Despite its contributions, this study has several limitations that should be acknowledged. First, the analysis is confined to bilateral agricultural trade between South Africa and 11 SADC member states, excluding other member states due to data availability. This selective sampling may limit the generalizability of the findings across the entire SADC region.

 

Reviewer Two

1. The author just describes the research in the abstract. Instead, the author could illustrate brief literature and research limitation, correspondingly highlight advantage of this paper, and manifest practical implications.

I have tried my level best to incorporate your suggestions given the constraints of a 200-word count recommended by the journal for an abstract (see changes below). I have particularly included briefly the literature aspect as well as the advantage or contribution of this paper. I have included limitations of the study in the limitations section found in section 5 of the revised version. Regarding the practical aspects of the paper, I am of the view that the section highlighted in green somewhat addresses this.

While tariff liberalization under regional trade agreements has progressed, non-tariff measures (NTMs) have emerged as a significant impediment to the realization of full trade potential, particularly in the agriculture sector where NTMs are especially prevalent and in the Southern African Development Community (SADC) where intra-regional trade is low. Despite the extensive available literature on this subject, the impact of NTMs on trade efficiency in SADC has hardly been explored. Against this background, this study estimates the impact of NTMs on the efficiency of South Africa’s bilateral agricultural trade with 11 SADC member states using data from 2011 to 2022 and a stochastic frontier gravity model. The average efficiency is found to be 45.6 percent implying that more than half of South Africa’s agricultural trade potential remains unrealized in the region due to inefficiencies. NTMs are found to be a source of inefficiency whose effect is larger than that of tariffs by a factor of 6. This result emphasizes an urgent need for harmonizing NTMs across SADC member states to reduce compliance costs which are associated with trade inefficiency. The study contributes to the literature by treating NTMs as man-made trade resistances that affect trade efficiency rather than trade volumes.

2. Overall, this paper is basically an empirical exercise. I respectfully suggest that the author could theoretically analyze and justify the finding (instead of just finding).

Thank you for the constructive criticism. My decision to exclude the theoretical sectional was solely based on the template of the journal which moves from introduction to materials and methods. I have however included the theoretical section highlighted in yellow which provides the basis of the empirical analysis. I have also provided a discussion that supports the a priori expectation of a disproportionate effect of NTMs compared to tariffs. All these additions are highlighted in yellow, section 2 of the revised manuscript.

3. The author claims “The study contributes to the literature by separating the effects of natural and man-made trade barriers in a regional and sector-specific context”.

• However, any effects can be classified as natural or man-made, so the study’s contribution can be vague.

I have provided more clarity to my claim. What I intended to communicate is that studies have mostly included natural (distance, common borders, common language etc) and man-made costs (tariff and NTMs) in the frontier specification. This has largely assumed that both types of costs affect frontier trade volumes. Contrary to this practice, this study treats natural trade costs as determinants of trade frontier volumes while man-made costs are treated as factors that affect trade efficiency. I have discussed this more explicitly in the theoretical section.

4. The author could improve the writing (with respect to typos in references).

I take this to refer to references which I believe will be addressed during the editorial process as indicated by the journal.

 

Reviewer 2 Report

Comments and Suggestions for Authors

Please see the report.

Comments for author File: Comments.pdf

Comments on the Quality of English Language

Please see the report.

Author Response

Dear Editor and Reviewers

Thank you for the opportunity to revise my manuscript. I am grateful for the valuable feedback provided by you and the reviewers. I have carefully considered each of the comments and have made substantial revisions to the manuscript in response.

Below, I provide a detailed response to the comments raised by the reviewers:

Reviewer 1.

1. First, please abbreviate SADC in the abstract.

I have done so, thank you.

2. While the paper highlights the importance of distinguishing between natural and man-made trade barriers, it does not discuss existing theoretical frameworks that explain how non-tariff measures (NTMs) specifically affect trade efficiency. I recommend enhancing the literature review to include the mechanisms through which NTMs obstruct trade, such as political economy issues, regulatory complexity, and institutional capacities.

Thank you for raising this. I have addressed the comments by adding a theoretical framework and empirical literature. The mechanisms are also explained in the theoretical framework. These changes are highlighted in yellow for ease of reference.

3. The choice of a stochastic frontier gravity model is suitable; however, the details regarding model specification, robustness checks, and variable selection are lacking. Discuss potential measurement errors and conduct sensitivity analyses to bolster validity. Moreover, consider incorporating more robust methods to evaluate the impacts of both non-tariff and tariff measures. The descriptive analysis cannot support claims about the impacts or causality of the independent variables, as outlined in Table 2. The authors should reanalyze the data using alternative methods, such as GMM or Driscoll-Kraay, etc. I suggest reviewing a relevant paper that utilized the stochastic frontier model for guidance (https://www.sciencedirect.com/science/article/pii/S0959652622000609).

Thank you for raising this. I have addressed the comment by adding a theoretical framework which provides a theoretical basis from which the empirical specification is derived. I have also performed a robustness exercise using an alternative estimator that is robust to potential endogeneity. Diagnostic tests have also been performed and reported to support the models used. The proposed GMM or Discroll Kraay required that I estimate the stochastic gravity model using a two-step approach which, as I stated in the methodology section is subject to a bias (see the references below). The one I applied in this study as a robustness check (Karakaplan, 2022) is consistent with the one-step approach recommended in literature and it addresses endogeneity using instruments similar to the GMM approach. I also reviewed the article you recommended. I felt, however, that the paper overly cites equations that may inadvertently confuse a non-technical reader without adding much value.

Wang, H.J. and Schmidt, P., 2002. One-step and two-step estimation of the effects of exogenous variables on technical efficiency levels. journal of Productivity Analysis, 18, pp.129-144.

Karakaplan, M.U., 2022. Panel stochastic frontier models with endogeneity. The Stata Journal, 22(3), pp.643-663.

I agree with you that descriptive analysis cannot support claims about the impacts or causality of the independent variables. You may have, however, missed the clarity provided in the original submission in the first paragraph of section 3 of the initial submission and section 4 of the revised. The table actually summarizes observation-specific marginal effects. To make this clearer, I have added a section of marginal effects in the methodology section which provides the clarity that readers may need in their understanding of Table 2.

This section presents the estimation results from the stochastic frontier gravity model, which decomposes total agricultural trade between South Africa and its SADC trading partners into potential trade (the frontier) and trade inefficiency. The model is estimated using panel data, where South Africa is the exporting country, and 11 SADC member states are the importers. The dependent variable is the total bilateral agricultural trade value. Robust standard errors are used to guide against potential heteroscedasticity. The results are presented in two tables. Table 1 presents the estimated baseline regression containing the frontier model and the inefficiency specification. Table 2 presents the unconditional marginal effects of tariffs and NTMs on trade inefficiency since the coefficients in Table 1 are conditional on the stochastic error term.

 

The added section in the methodology is as follows.

 

3.2 Marginal Effects

In stochastic gravity frontier model of international trade, the relationship between tariff and NTMs and the inefficiency term is inherently non-linear due to the composite error structure which comprises both the random noise and the inefficiency component. As a result, the estimated coefficients in the inefficiency equation do not directly indicate the marginal effects of the covariates on inefficiency. Instead, the marginal effects must be computed to account for the interaction between each covariate and the distributional assumptions of the inefficiency term. This is particularly important since the conditional inefficiency term is derived from a non-linear function involving probability density and cumulative distribution functions. Against this background, we computed the marginal effects post estimation following the derivation proposed by Wang (2002).

where  is a vector of variables affecting trade efficiency (i.e., costs arising from tariff and NTMs),  and  and  are the corresponding coefficients. Wang (2002) demonstrates that the marginal effect of a  variable on trade inefficiency is nonmonotonic implying that the impact depends on the values of exogenous and may vary across observations. It is therefore necessary to compute observation-specific marginal effects and summarize to observe the average impact of tariff and NTMs.

 

4. Although the paper advocates for harmonization, it provides limited insight into practical challenges such as sovereignty issues, capacity constraints, and political resistance. I recommend adding discussions on how these obstacles could be overcome. For the literature review, please include recent regional studies, WTO/UNECA reports, and contemporary empirical research on trade facilitation to strengthen your analysis. Please add future research directions.

Thank you for these comments and suggestions. I have enhanced the discussion section to accommodate this. As you will notice in yellow sections, I have particularly added a discussion around sovereign issues and how such challenges may be overcome. I have also added contemporary literature including a WTO report in the empirical literature review which similarly found a relatively large negative effect of NTMs on trade. I have also added a limitations section which includes areas for further research.

Authors should address the following questions:

1. Which specific non-tariff measures contribute most to trade inefficiency?

I have provided a clearer explanation of how this study proxied NTMs. This is found under data description in the methodology section, and it is highlighted in yellow. I have also added, in the limitations section, a paragraph that explains how, despite the strength of using trade costs data, the study is unable to separate the effects of different types of NTMs. I am of the view that the benefit of using trade costs arising from NTMs (as a proxy variable) outweighs the above limitation to the extent that they provide a way of directly gauging the burden borne by exporters.

1. How can SADC countries collaborate to harmonize NTMs and enhance trade flow? Is there proof that some countries or policies are better at reducing negative impacts?

Thank you for raising an important practical question. I have added some paragraphs in the discussion addressing this in line with your recommendation.

1. How are non-tariff measures quantified in your analysis, and what limitations exist in this measurement approach?

This is explained in the methodology section as indicated below.

In the inefficiency equation, the study includes two variables capturing trade costs arising from tariffs and NTMs. Rather than focusing on specific NTMs such as sanitary and phytosanitary measures, technical barriers to trade, or import/export licenses, the study employs aggregate trade cost data derived from the Economic and Social Commission for Asia and the Pacific (ESCAP)-World Bank trade cost database. This approach offers several advantages. First, using trade costs as a variable captures the overall economic impact of NTMs rather than using specific measures. While notifications of NTBs or SPS measures indicate the presence of regulatory restrictions for instance, they do not provide direct information on the cost implications of these barriers for traders. In contrast, trade costs data, measured in ad valorem terms, quantify the actual economic burden imposed by NTMs, thus allowing for a better assessment of their impact on trade flows (Chen & Novy, 2012). In addition, this methodology facilitates a direct comparison of the relative costs associated with tariffs and NTMs (Anderson & van Wincoop, 2004).

I have included the limitations under the limitations section.

Second, the study employs aggregate NTM trade costs data without distinguishing between different types of NTMs (e.g., sanitary and phytosanitary measures, technical barriers to trade, import/export licenses). As a result, the study is unable to provide specific policy recommendations targeted at particular NTM categories that may have distinct effects on trade efficiency. Future studies could consider disaggregating NTMs to better capture their heterogenous effects.

 

1. How does your empirical model account for potential endogeneity between NTMs and trade flows, ensuring that observed relationships reflect causal impacts rather than reverse causality or omitted variables?

Thank you for the comment. I have addressed potential endogeneity as alluded to earlier using the instrumental variable stochastic frontier model proposed by Karakaplan (2022). The results of this robustness exercise are presented in Table 3. This addition is highlighted in yellow in the results section.

1. What practical challenges and political considerations arise from harmonizing NTMs across SADC member states, and how do your findings on agricultural trade efficiency apply to other sectors or regions within SADC, impacting regional integration strategies?

Thank you for raising this. Kindly see the discussion section which has been enhanced to address this in line with your suggestions which I agree with. Regarding the applicability of the results to other SADC, I have cited this as a limitation (see below).

Despite its contributions, this study has several limitations that should be acknowledged. First, the analysis is confined to bilateral agricultural trade between South Africa and 11 SADC member states, excluding other member states due to data availability. This selective sampling may limit the generalizability of the findings across the entire SADC region.

 

Reviewer Two

1. The author just describes the research in the abstract. Instead, the author could illustrate brief literature and research limitation, correspondingly highlight advantage of this paper, and manifest practical implications.

I have tried my level best to incorporate your suggestions given the constraints of a 200-word count recommended by the journal for an abstract (see changes below). I have particularly included briefly the literature aspect as well as the advantage or contribution of this paper. I have included limitations of the study in the limitations section found in section 5 of the revised version. Regarding the practical aspects of the paper, I am of the view that the section highlighted in green somewhat addresses this.

While tariff liberalization under regional trade agreements has progressed, non-tariff measures (NTMs) have emerged as a significant impediment to the realization of full trade potential, particularly in the agriculture sector where NTMs are especially prevalent and in the Southern African Development Community (SADC) where intra-regional trade is low. Despite the extensive available literature on this subject, the impact of NTMs on trade efficiency in SADC has hardly been explored. Against this background, this study estimates the impact of NTMs on the efficiency of South Africa’s bilateral agricultural trade with 11 SADC member states using data from 2011 to 2022 and a stochastic frontier gravity model. The average efficiency is found to be 45.6 percent implying that more than half of South Africa’s agricultural trade potential remains unrealized in the region due to inefficiencies. NTMs are found to be a source of inefficiency whose effect is larger than that of tariffs by a factor of 6. This result emphasizes an urgent need for harmonizing NTMs across SADC member states to reduce compliance costs which are associated with trade inefficiency. The study contributes to the literature by treating NTMs as man-made trade resistances that affect trade efficiency rather than trade volumes.

2. Overall, this paper is basically an empirical exercise. I respectfully suggest that the author could theoretically analyze and justify the finding (instead of just finding).

Thank you for the constructive criticism. My decision to exclude the theoretical sectional was solely based on the template of the journal which moves from introduction to materials and methods. I have however included the theoretical section highlighted in yellow which provides the basis of the empirical analysis. I have also provided a discussion that supports the a priori expectation of a disproportionate effect of NTMs compared to tariffs. All these additions are highlighted in yellow, section 2 of the revised manuscript.

3. The author claims “The study contributes to the literature by separating the effects of natural and man-made trade barriers in a regional and sector-specific context”.

• However, any effects can be classified as natural or man-made, so the study’s contribution can be vague.

I have provided more clarity to my claim. What I intended to communicate is that studies have mostly included natural (distance, common borders, common language etc) and man-made costs (tariff and NTMs) in the frontier specification. This has largely assumed that both types of costs affect frontier trade volumes. Contrary to this practice, this study treats natural trade costs as determinants of trade frontier volumes while man-made costs are treated as factors that affect trade efficiency. I have discussed this more explicitly in the theoretical section.

4. The author could improve the writing (with respect to typos in references).

I take this to refer to references which I believe will be addressed during the editorial process as indicated by the journal.

 

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Please review for any grammatical and stylistic errors before finalising the production of the manuscript. 

Reviewer 2 Report

Comments and Suggestions for Authors

The author substantially improves the paper.