This article makes a much-needed contribution to advancing the research literature on loneliness. By evaluating psychometric properties of existing loneliness measurement tools, the study will yield important implications for policies and practices aiming to curb the growing loneliness epidemic. I applaud the authors for their methodological rigor. The text is well written and the authors provide an interesting summary of the findings from the exploratory fold. Furthermore, they have done a commendable job in addressing the reviewers’ previous comments. I have attached several additional recommendations that may strengthen the clarity and relevance of the article to end-users.

I have reviewed the authors' responses to my comments. Thank you for having addressed them. I am happy to approve this manuscript to proceed. Good luck with the research and Stage 2 write up!

The authors provided a comprehensive revision that addressed all of my previous comments. I have no further comments to add.

This article makes a much-needed contribution to advancing the research literature on loneliness. By evaluating psychometric properties of existing loneliness measurement tools, the study will yield important implications for policies and practices aiming to curb the growing loneliness epidemic. I applaud the authors for their methodological rigor. The considerations posed throughout the text are thoughtful and the paper is well-written. I have enclosed several key recommendations that may strengthen the quality, clarity, and relevance of the article to end-users. These comments are summarized in the attached document. I have also attached a revised manuscript with additional feedback in track changes and the comment bar.

1A. The scientific validity of the research question(s)

The current research aims to look at performance of two loneliness scales, the DJGLS-6 and T-ILS, in data sets collected from samples representing 27 EU member states. The research questions are important and the findings will allow the readers to determine 1) factor structure of the 2 scales, 2) construct validity based on correlations with related constructs, and 3) whether and for which countries can the scales be used and for cross-country comparisons. I particularly think #3 is very important. 

1B. The logic, rationale, and plausibility of the proposed hypotheses (where a submission proposes hypotheses)

The hypotheses are logical and rational. The authors also indicate where they are sure or not sure about the apriori hypotheses. 

1C. The soundness and feasibility of the methodology and analysis pipeline (including statistical power analysis or alternative sampling plans where applicable)

The method of data collection, including translation of the original English version, was explained. However, I would like the author to add in details related to translation process; for example, were texts translated verbatim, or were there steps to also gather cultural input from native-speakers of each country? I understand that data has been collected, so I am only asking for clarification so future research can look for ways to improve, in the case that texts were only translated verbatim. 

1D. Whether the clarity and degree of methodological detail is sufficient to closely replicate the proposed study procedures and analysis pipeline and to prevent undisclosed flexibility in the procedures and analyses

Yes. 

1E. Whether the authors have considered sufficient outcome-neutral conditions (e.g. absence of floor or ceiling effects; positive controls; other quality checks) for ensuring that the obtained results are able to test the stated hypotheses or answer the stated research question(s).

I would like the authors to consider more the descriptive statistics of the scales. There was a discussion around how to deal with 3-point scales and not to treat them as continuous scales. However, what if there is a ceiling effect where most participants in any specific countries are either on the very high or low end of the response scales, would this affect the planned analyses in anyway (I do not know because I am not an expert in these analyses so this is a question in case it needs to be considered). In that case, will data need to be recoded for that country, for example?

Overall, I think this is a very good and well thought-out research plan and I look forward to reading the findings of this work. 

Dr. Thuy-vy Nguyen

I would like to thank the authors and the editor for the opportunity to review this Stage 1 report. I found the report very clear, well-written, and exceptionally detailed. The research addresses an important question with commendable rigor. Below, I present my detailed evaluation, structured in alignment with the criteria delineated by PCI.

1A. The scientific validity of the research question(s): The research question focuses on establishing the psychometric properties of short scales to measure loneliness across EU countries. This question is of high practical and scientific importance. The background literature is well described and shows a clear need for this research. The psychometric approach is also clearly described and aligns well with the research question.

The study seems to have been conducted in accordance with ethical research standards. However, a statement about independent ethics review is missing. It is recommended to include a statement regarding independent ethics review to enhance the report's comprehensiveness.

1B. The logic, rationale, and plausibility of the proposed hypotheses: The study is largely descriptive with a focus on establishing psychometric properties of established self-ratings scales of loneliness across EU countries. The proposed psychometric measures appear well suited towards this aim. The rationale for establishing these psychometric properties follows logically from the extensive review of previous studies from different EU countries.

1C. The soundness and feasibility of the methodology and analysis pipeline: As I am not a statistician, I am unable to comment on the finer details of the analysis. From my knowledge, the description of the proposed method appears very detailed and well-suited for the aims of the study. The cross-validation approach with pre-specified exploratory and confirmatory analyses in different folds of the data is particularly strong, in my view.

1D. Whether the clarity and degree of methodological detail is sufficient to closely replicate the proposed study procedures and analysis pipeline and to prevent undisclosed flexibility in the procedures and analyses: The description of the methodology is extensive and should allow for the replication of the findings. Making the code available prior to the analysis is another strong point. However, this code could be better documented. While the input and output parameters are clearly defined, the purpose and parameters of various helper functions are less clear. Improving the documentation, especially regarding the purpose and parameters of various helper functions, would enhance the code's clarity. It would also be helpful to know if the code was tested with simulated data.

1E. Whether the authors have considered sufficient outcome-neutral conditions: The aim of the study is largely exploratory. By incorporating both exploratory and confirmatory steps, the authors consider all possible factor structures for the loneliness measures. Further, the authors plan to provide objective measures of model fit in both the exploratory and confirmatory analyses. This approach ensures a comprehensive understanding of potential outcomes by providing objective measures of model fit in both the exploratory and confirmatory analyses.