The present study had some strengths. A total of 15 epidemiologic studies were included in this systematic review and meta-analysis, which improved the statistical power and reported more reliable estimates compared with any individual study. Various subgroup analyses according to key characteristics were performed, which provided a comprehensive and in-depth evaluation of the relationship between male baldness and prostate cancer. Sensitivity analyses and Galbraith plot were carried out to confirm the robustness of pooled risk estimate. The effect size estimates were extracted from fully adjusted models in each study, which might minimize the potential confounding. No obvious publication bias was observed in Begg test or Egger test.
However, several important limitations should also be acknowledged in this study. First, statistical significantly heterogeneity was detected across included studies (P < .078 for heterogeneity, I2 = 36.4%), which might affect the robustness of the pooled risk estimates. We removed the studies that contributed to the heterogeneity via the Galbraith plot and recalculated the combined RR, which did not remain significant (RR: 0.99, 95% CI 0.94–1.04). This result implied that the overall analysis was not substantially affected by the heterogeneity. Second, most of the eligible studies were case–control studies, which were less conclusive than cohort studies and might introduce the select bias and recall bias. Third, although the majority of the included studies adopted the Norwood–Hamilton scale to assess the baldness, differences in exposure assessment still existed, which might distort the combined risk estimates. Finally, gray literature (e.g., conference abstracts) and studies reported in languages other than English or Chinese were less likely to be retrieved, which might lead to a certain degree of publication bias.
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