From studies providing results on success of blinding, Bang’s blinding index (BI)  was calculated with 95% confidence intervals (CI) to further investigate the association between information on placebo and effectiveness of blinding in trials. Bang’s BI yields a value between − 1 and 1 for each arm in a study, providing the proportion of the potentially unblinded beyond chance . BI of 1 would mean that everyone guessed correctly, and 0 means guesses were random, whereas − 1 would mean that all guesses were incorrect (e.g. all participants in the placebo arm answered that they believe they received real treatment). The number of participants in each response of each arm was extracted from the included studies and more information was sought from the original authors if necessary. If a study measured outcome at more than one time point, the data measured at the end of the last intervention or at the closest were extracted. Based on BIs, each study was classified into one out of nine possible blinding scenarios: in brief, random guess, unblinded or opposite guess is given to each arm in a study, and then, nine blinding scenarios are possible [10, 14, 15]. The threshold for blinding classification was random guess for − 0.2 < BI < 0.2; unblinded for BI ≥ 0.2; and opposite guess for BI ≤ − 0.2 . It should be noted that throughout this paper, the term “unblinded” generally means “more correct guess,” not broken blinding literally.