Fig. 2

Assessment of risk of bias of 24 reviewed articles using the Appraisal tool for Cross-Sectional Studies (AXIS). a Distribution of per-item scores, indicating, for each item, the number of articles satisfying its definition. A detailed description of AXIS items and shortcomings of individual articles is presented in Additional file 1: Tables S1 and S2. b Distribution of aggregate AXIS scores computed as the number of items satisfied by any given reviewed article. Trichotomisation of the theoretical range 0–11 leads to the risk of bias being judged as high (0–3), moderate (4–7), or low (8–11)