An analysis was made of data from over 4000 postepisiotomy, uterine cramping, and postsurgical patients complaining of moderate or severe pain. They had received 325, 650, or 1300 mg aspirin or placebo while they were subjects in 10 analgesic clinical trials. On the average, for the same verbally expressed pain intensity level and the same treatment, more relief was obtained by a patient with uterine cramping than one with episiotomy pain, who in turn obtained more relief than a patient with surgical pain. A new mathematical model which characterizes the probability that an analgesic provides complete relief as a function of dose, severity of pain intensity, and pain etiology is developed. The model utilizes the data itself to estimate the numerical score corresponding to verbal pain intensities. The results indicate that the numerical score quantifying severe surgical pain is 1.4 times greater than the score for severe episiotomy pain, which in turn is 3.2 times greater than the score for severe uterine cramping. Clinical trials must be designed to take these differences into account. Also, clinicians must be cognizant of such differences when choosing among drugs and dosages for patients with different pain intensity and etiology.