The data are from a multicenter randomized clinical trial to compare test and control treatments for the management of chronic pain, and they have had previous consideration in Stokes et al. (2000, chap. 13).
data(cpain)A data frame with 193 observations and 4 variables
treata factor with levels active and placebo for treatment
responsea factor with five levels poor, fair, moderate, good and excel for pain status after treatment for 4 weeks
centera factor with two levels I and II for two centers
diagnosisa factor with four levels A, B, C, and D for diagnoses
Stokes, M. E., Davis, C. S., and Koch, G. G. (2000), Categorical Data Analysis using the SAS System, Cary: SAS Publishing.