SAMPLE-SIZE DETERMINATION FOR AN EXPONENTIAL SURVIVAL MODEL WITH AN UNRESTRICTED COVARIATE

We derive formulae for estimating sample size and power for detecting the effect of an unrestricted covariate on survival time. These are us eful in designing survival studies with different patterns of recruitm ent and follow-up when survival time is exponentially distributed. We use the asymptotic covariance matrix, conditional expectation and Tayl or's expansion techniques to develop these formulae. Computer simulati ons indicate that the asymptotic approximations used in developing the formulae are good over a range of parameter values and different patt erns of recruitment and follow-up that are relevant to survival studie s.