The Relative Age Effect Beyond Geography: Evidence of a Shared Pattern in Senior International Football

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The study is methodologically more ambitious than typical RAE papers; however, several aspects of the analysis and its reporting are insufficiently developed to fully support the conclusions. Therefore, the following revisions are necessary to strengthen the methodological rigor, clarity, and overall interpretation of the findings

Positioning relative to prior work is incomplete. The introduction and discussion would benefit from a broader engagement with the existing literature on relative age effects, particularly studies examining confederation-level differences and RAE patterns in senior World Cups. I suggest considering some of the key studies in this area, including the work of Steingröver et al. (2017) and Costa et al. (2015), as well as the foundational theoretical review by Musch and Grondin (2001). Incorporating these and other relevant studies would help to better contextualize the present findings within the existing literature.

The description of the cluster analysis is too brief to evaluate or replicate. The algorithm, distance metric, number of clusters tested, and any validation criteria are not reported. Similarly, the CLMM model specification (fixed effects, random effect structure, link function, estimation method) is not fully described.

Statistical reporting and methodological transparency require further clarification. I recommend reporting odds ratios with 95% confidence intervals for all predictors included in the cumulative link mixed model (CLMM), together with the variance component of the random effect and its confidence interval. In addition, the methodology of the cluster analysis should be described in greater detail, including the clustering algorithm, distance metric, number of clusters evaluated, and the criteria or validation metrics used to determine the optimal cluster solution. A supplementary figure, such as a dendrogram or silhouette plot, should also be provided to support the interpretation and validation of the clustering results. Finally, the source of the playing-position variable and the procedure used to categorize playing positions should be clearly described.

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The academic article has the following objective: This study examined the RAE in senior international football using a cross-sectional sample of 1,248 players from the 48 national teams participating in the 2026 FIFA World Cup.

The authors are requested to address the following items:

1. In the manuscript, it is described that “the overall distribution of birth quarters ... significantly departed from the equal distribution of 25% per quarter”, and it is also acknowledged in the limitations that this reference does not take into account the seasonality of births. In this regard, to study a Relative Age Effect (RAE), comparing players with a uniform distribution may be an initial approach, but it does not necessarily constitute a valid population reference, because births are not uniformly distributed throughout the year and, in addition, the 48 countries have different seasonal birth patterns. This directly affects χ² = 18.80, p < .001 and, by extension, the statement that the analysis “confirms” the RAE.
2. Given the above, national birth distributions should ideally be incorporated as a reference, as well as the corresponding population data from the players’ countries of origin, or at least sensitivity analyses using different expected distributions.
3. The manuscript uses the following terms interchangeably: RAE; birth-quarter distribution; early-born players; H1 representation; selection bias. These concepts are related, but they are not equivalent.
4. No assessment of the proportionality assumption is reported. The article uses a Cumulative Link Mixed Model, but does not report any test or diagnostic of the fundamental assumption of the ordinal model. It should be specified whether the proportional odds assumption, parallel slopes assumption, adequacy of the logit link, residual diagnostics, influential observations, and overall model fit were assessed.
5. The statement “Independent predictor” may suggest predictive capacity or causal independence that the design does not allow to be demonstrated in the research under evaluation.
6. The conclusion used by the authors regarding a “shared international pattern” is too strong. The study demonstrates, at most, that there is an unequal distribution, that no statistically significant association with confederation was found, that the variance between teams is low, and that the clustering did not find strong separation. However, it does not necessarily follow from this that there is a “structural bias in talent identification and development systems”, because the talent identification and development systems were not measured. Therefore, the conclusion should be much more cautious.
7. Although N = 48 countries seems reasonable for an exploratory analysis, each country has only 26 players (the cluster size is small). For example, one player represents 1/26 = 3.85% of the team. Therefore, apparently large differences in national percentages may arise from changes involving only a few players. This should be explicitly discussed when interpreting the map and the clustering.
8. The article reports: χ² = 18.80, df = 3, p < .001, but does not provide an effect size.
9. National proportions, particularly with n = 26 per team, should be accompanied by confidence intervals when they are used for descriptive comparisons.
10. It is mentioned that missing values were assessed, but it is not specified: how many missing values there were for each variable, which variables were ultimately included in the CLMM, whether a complete-case analysis was performed, whether imputation was used, whether observations were excluded, or the impact on N.
11. The issue of multiplicity is not adequately addressed. The following are not mentioned: correction for multiple testing, FDR, Bonferroni, or Holm.
12. The Declaration of Helsinki is cited as an ethical reference. However, the study uses exclusively publicly available secondary data.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have addressed all of the requested revisions and, as a result, have significantly improved the quality and overall presentation of the manuscript. In my opinion, the manuscript has now been adequately revised and is suitable for publication in the journal in its current form.

Reviewer 2 Report

Comments and Suggestions for Authors

Most of the issues raised have been resolved.