BI 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