Sample-size calculations for the Cox proportional hazards regression modelwith nonbinary covariates

This paper derives a formula to calculate the number of deaths required for a proportional hazards regression model with a nonbinary covariate. The me thod does not require assumptions about the distributions of survival time and predictor variables other than proportional hazards. Simulations show t hat the censored observations do not contribute to the power of the test in the proportional hazards model, a fact that is well known for a binary cov ariate. This paper also provides a variance inflation factor together with simulations for adjustment of sample size when additional covariates are in cluded in the model. (C) Elsevier Science Inc. 2000.