Pre-trial evaluation of the potential for unblinding in drug trials: a prototype example

Blinding is an important design feature of randomised trials that may reduce bias in the results, compared to the situation where blinding is not possible or is not maintained. The literature provides some guidance for the evaluation of blinding in ongoing or completed studies, but the question of pre-trial assessment of the potential for unblinding has not been addressed. This paper describes the design and analysis of a prototype experiment for the pre-trial assessment of blinding in a drug trial. This work was motivated by a trial using antibiotic therapy, in which the investigators were concerned about the possibility of subjects being able to differentiate active medication from placebo, and thus become unblinded to their treatment assignment. A small experiment was mounted in which participants had to divide a random mixture of tablets into two groups. Statistical methods were developed to calculate the probability of a given number of similar tablets being classified into the same group by chance, with a modification to allow for some participants having constrained their responses to have equal numbers of tablets in each group. Differentiation of tablets by taste (the initial concern of the investigators) was not statistically different from chance. A smaller set of data on differentiation by appearance (a possibility not originally considered) had borderline statistical significance. After reviewing all these results, the investigators decided to proceed with the study without modifying the tablets, in part because subjects in the study would be unlikely to compare the two types of medication side-by-side. Our results suggest that blinding might sometimes be compromised in unexpected ways. Whenever possible, we suggest that similar and larger such experiments be carried out before the trial to assess whether blinding might be compromised. The methods proposed here could easily be adapted to evaluate the results of such experiments.