Accuracy of subjective measurements made with or without previous scores: an important source of error in serial measurement of subjective states.

Pain severity was assessed on visual analogue scale in 4 groups of patients receiving long-term therapy. A comparison was made between pain measurements made with and without access to initial scores. The differences between these measurements increased with the duration of treatment. Patients tended to overestimate their pain severity when previous scores were not available. It is suggested that initial scores should be made available when serial measurements of pain are made in long-term experiments.