Pain is the most common reason that patients seek medical help. Pain continues to present difficulties, however, as regards both its definition and the methods for accurately measuring it. Our inability to quantify pain adequately has handicapped the comparison of different analgesic drugs, as well as the treatment of pain in general.1 One of the main problems in measuring pain is that the word pain refers to a great variety of sensory experiences rather than a single specific sensation varying only in intensity.2 A number of more sophisticated ways of measuring pain have therefore been proposed, such as the McGill-Melzack pain questionnaire,2 measurements based on signal detection theory,3 and functional measurement scaling.4
Another difficulty in accurately assessing the potency of analgesic drugs is the sizable number of patients who exhibit a substantial placebo response. The importance of the placebo response in the treatment of Pain