A new study in the Annals of Internal Medicine asked how often placebo-controlled medical studies made clear what the placebo was. The abstract says:
Most studies did not disclose the composition of the study placebo.
Which may give the wrong impression. About 90% of the studies they looked at did not say what the placebo was.
The paper illustrates the problem with an example:
In one of these studies [where the placebo was not described], the authors commented that “The lack of any overall effect in patients with myocardial infarction might be related to the unexpectedly low mortality rate in the placebo group.” The possibility that the placebo composition may have influenced this “unexpectedly low mortality” was apparently not considered.
Thanks to Gunnar Schröder.