A meta-epidemiological study has reported that well maintained blinding resulted in a smaller effect of intervention compared to non-blinded trials . In the present study, however, pooled effect size of trials in DD category, which showed better blinding results than FD category, was greater than that of FD category. There could be two possible explanations. First, we may suspect that informing the presence of placebo acupuncture in detail lowered the participants’ expectation and as a result, possibly undermined the effect of real acupuncture in studies in FD category, whereas the participants’ expectation was maintained or enhanced in DD category and the trials favored acupuncture to a larger extent. This is supported by a pooled analysis from four, large population based German RCTs which demonstrated higher expectation was associated with a better outcome regardless of given treatment . Considering that our study was not designed to directly measure participants’ expectation of treatment, however, we cannot estimate to what extent expectation contributed to greater effect size from the studies in DD category than those in FD category. Another explanation is that the treatment effect in DD category is rather overestimated due to investigator’s bias. Not only the patients but also trialists have expectations, wittingly or unwittingly . Considering the fact that more studies in DD/MI category originated from Asia, which is an acupuncture friendly region where researchers are eager to prove its efficacy, than FD category, it may be trialists’ expectation on top of the participants’ expectation that resulted in such higher estimation. Besides, when we evaluated whether outcome assessor was blinded to the intervention in the included studies, the proportion of studies with appropriate blinding for outcome assessor was slightly lower in DD category (20 out of 25, 80%) compared to FD category (33 out of 40, 82.5%). This too supports investigator bias may have possibly played a part in overestimation of treatment effect in the studies of DD category. Nonetheless, the fact that only small number of studies were included here and that this is merely observational in nature allows for an interpretation in a narrow context. To elucidate complicated relationships between PILs, blinding, and outcomes of a trial, further studies designed to compare the links between those are warranted for more discussion.