2.4. Quality Assessment

The quality of the studies was assessed independently by two authors (D.G., M.K.S.) according to a modification of the Newcastle–Ottawa Quality Assessment Scale (NOS) separately for case–control [16] and for cross-sectional studies (each study type had its modification of NOS) [17] as presented in Table 1.

Quality assessment of studies included in the systematic review using a modified Newcastle–Ottawa Quality Assessment Scale for case–control studies [16] and cross-sectional studies [17].

Case–control studies (range of total points 0–9): 0 to 3 points—low-quality study, 4 to 6 points—medium-quality study, and 7 to 9 points—high-quality study. Cross-sectional studies (range of total points 0–10): 0 to 3 points—low-quality study, 4 to 6 points—medium-quality study, and 7 to 10 points—high-quality study.

We used a modified version of NOS in order to include the specificity of the subject of the review; for example, for case–control studies we decided to award points not only for the validated tool (as in the original version) to assess PMS, but also for a well-described one, due to the reasons mentioned previously. Due to the fact that the potentially important factors contributing to PMS are not definitely defined, we assigned a point for including any of them (such as body mass index or age of menarche) in the adjustment of the study results. The NOS is recommended by the Cochrane Collaboration for use in nonrandomized studies, as a validated, quick and adaptable tool for quality assessment. However, it should be underlined that it is not validated for cross-sectional studies and some researchers indicate problems with its poor agreement in point awarding. The NOS evaluates three parameters (selection, comparability, and exposure or outcome depending on the type of the study) divided across eight (for case–control studies) or seven (for cross-sectional studies) specific items [16,17]. Detailed rules, according to which points were awarded in individual categories, are shown in Supplementary Materials.