The Need for Blinding?
Schulz KF, Chalmers I, Hayes R, Altman DG. Empirical Evidence of Bias: Dimensions of Methodological Quality Associated With Estimates of Treatment Effects in Controlled Trials. JAMA 1995;273:408-412.
based on 33 meta-analyses of 250 primary trials involving a total of 62,091 participants and 12,030 outcome events.