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Nonproportional hazards can often be expressed by extending the Cox model to include time varying coefficients; e. g. , for a single covariate, the hazard function for subject i is modelled as exp β (t) Zi (t). A common example is a treatment effect that decreases with time. We show that the function βi (t) can be directly visualized by smoothing an appropriate residual plot. Also, many tests of proportional hazards, including those of Cox (1972), Gill & Schumacher (1987), Harrell (1986), Lin (1991), Moreau, O'Quigley & Mesbah (1985), Nagelkerke, Oosting & Hart (1984), O'Quigley & Pessione (1989), Schoenfeld (1980) and Wei (1984) are related to time-weighted score tests of the proportional hazards hypothesis, and can be visualized as a weighted least-squares line fitted to the residual plot.
Grambsch et al. (Mon,) studied this question.