BI values for each study were calculated with their 95% CI (Fig. 2). Average BI values for real acupuncture groups in FD and DD/MI categories were 0.42 and 0.41, respectively. In other words, 42% (95% CI 0.23 to 0.62) of participants in the real acupuncture groups who were given full disclosure of placebo information and 41% (95% CI 0.21 to 0.61) in the real acupuncture groups from the studies with no disclosure (DD/MI category) correctly guessed that the given treatment was real acupuncture beyond chance (Fig. 2a and b). On the other hand, 16% of the participants in the placebo group in the studies with full information disclosure believed that they received real acupuncture beyond chance (− 0.16, 95% CI −0.39 to 0.06, Fig. 2c). In the groups without adequate information on placebo acupuncture, 21% of the participants in the placebo group guessed they were given real acupuncture (− 0.21, 95% CI −0.47 to 0.05, Fig. 2d). There was no significant difference in BI values of both real and placebo acupuncture groups between the two categories, i.e. disclosure vs. no disclosure (real acupuncture, p = 0.92, independent t-test; placebo acupuncture, p = 0.38, Mann-Whitney U test).Fig. 2BI values by PIL categories. a BI values of real acupuncture group from FD category; b BI values of real acupuncture group from DD/MI category; c BI values of placebo acupuncture group from FD category; d BI values of placebo acupuncture group from DD/MI category. Negative values indicate opposite guessing of allocated arm, 0 refers to random guessing, and positive values indicate correct guessing of allocated arm (e.g. guessed real acupuncture when assigned to real acupuncture). This study defined BI value between − 0.2 and 0.2, presented in the shaded area in each graph, as random guess . BI, blinding index; DD, deceptive disclosure of placebo acupuncture; FD, full disclosure of placebo acupuncture; MI, missing information on placebo acupuncture; PIL, participant information leaflet