A graphical method to assess treatment-covariate interactions using the Cox model on subsets of the data

We introduce the subpopulation treatment effect pattern plot (STEPP) method , designed to facilitate the interpretation of estimates of treatment effec t derived from different but potentially overlapping subsets of clinical tr ial data. In particular, we consider sequences of subpopulations defined wi th respect to a covariate, and obtain confidence bands for the collection o f treatment effects (here obtained from the Cox proportional hazards model) associated with the sequences. The method is aimed at determining whether the magnitude of the treatment effect changes as a function of the values o f the covariate. We apply STEPP to a breast cancer clinical trial data set to evaluate the treatment effect as a function of the oestrogen receptor co ntent of the primary tumour. Copyright (C) 2000 John Wiley & Sons, Ltd.