The reliability analysis revealed that the Kappa was poor to fair in some items on AMSTAR. As kappa coefficient shows the proportion of agreement beyond that expected by chance alone, it is a popular measure for chance-corrected nominal scale agreement between two raters [5, 8] ; however, if distribution of item responses is skewed or over concentrated on either the "yes" or the "no" category, the kappa coefficients will become unstable and invalid, and no longer suitable for measuring agreement between two raters, so new methods for calculating valid agreement coefficients are needed to explore in future study. Such items shown in our study included item 1 "was an 'a priori' design provided," item 6 "were the characteristics of the included studies provided," item 7 "was the scientific quality of the included studies assessed and documented," and item 8 "was the scientific quality of the included studies used appropriately in formulating conclusions"; however, the relatively low kappa in item 5 and item 9 cannot be explained by the limitation of kappa statistic.