Grana et al. (2002) report the results from a nonrandomized clinical trial investigating a novel radioimmunotherapy in malignant glioma patients. The overall survival, that is, the time from the beginning of therapy to the disease-caused death of the patient, is compared for two groups of patients—one, the standard therapy plus the novel therapy; and the other, the standard therapy alone.

In addition to the treatment groups a number of other explanatory variables are recorded, namely age, gender, and histology. The data are given in exer_73.txt. Fit a Cox model to the data and find a CI for the treatment effect conditional on age, gender, and histology.