2.3 Assessment of methodological quality
We used the Newcastle-Ottawa scale (NOS) [20] for assessing the quality of the included studies in this meta-analysis. Using this scale system, each included study was judged on three broad perspectives: the selection of the study groups, the comparability of the groups, and the ascertainment of outcome of interest. High-quality studies were identified with a NOS score of five or more. Whereas those with less than a score of five were considered as low-quality studies.