Survival analysis sample size calculations

Total sample size may be evaluated for comparing hazard rates (per unit time) using this [attachment:coxsamp.xls spreadsheet] which uses a simple formula taken from Collett (2003) [http://stats.stackexchange.com/questions/7508/power-analysis-for-survival-analysis illustrated here] corresponding to a group regression estimate (ratio of hazards) in a Cox regression model. Alternatively the effect size can be expressed in terms of ratios of group survival rates as used by the power calculator given [http://www.stattools.net/SSizSurvival_Pgm.php here.]

In particular Collett(2003) gives the total sample size, d, required as

d = $$\frac{(z_text{a/2} + z_text{b/2})^{text{2}}{p(1-p)log(hr)}text{2}}$$

for two-sided type I error, a, power 1-b, event rate in population p and hazard ratio, hr.

Reference

Collett, D (2003) Modelling Survival Data in Medical Research Second Edition. Chapman and Hall:London