In randomised controlled trials, an increasingly used test of the 'blindness' of assessors is to have them guess the group to which each participant has been allocated. Because assessors are usually aware of the trial hypotheses, we predicted that trial participants who showed the greatest improvement would be assumed by an assessor to have been in the 'preferred intervention' group.
Data were derived from a trial in recent-onset rheumatoid arthritis comparing cognitive behavioural therapy plus routine care with routine care alone.
Although in this trial assessor blindness was demonstrated, patients 'guessed' by the assessor to be in the cognitive therapy group showed significantly greater improvements than those predicted to have received routine care alone in variables predicted to change in the study protocol.
These results indicate that even if an assessor's guesses about patient group allocations are more accurate than expected by chance, this would not necessarily demonstrate failure of blinding. This casts further doubt on the validity of forced choice paradigms in testing the integrity of blinding in clinical trials.