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SUMMARY The regression model for proportional hazards (Cox, 1972) is extended to allow (approximately) for time-varying covariate coefficients. This is particularly appropriate for cancer mortality studies where the effect of a covariate measured at one point of time decays as time progresses. A smoothed hazard function estimate for this extended model is derived using penalized maximum likelihood. A case-history is given concerned with recurrence of carcinoma of the vulva. The basic model does not fit very well, but the extended model fits well.
Anderson et al. (Fri,) studied this question.