  In a stratified clinical trial design with time to event end points,
stratification factors are often accounted for the log-rank test and the Cox
regression analyses. In this work, we have evaluated the impact of inclusion of
stratification factors on the power of the stratified log-rank test and have
compared the bias and standard error in HR estimate between multivariate and
stratified Cox regression analyses through simulation. Results from our
investigation suggests that both failing to consider stratification factor in
presence of their prognostic effect and stratification with smaller number of
events may substantially reduce the power of the log-rank test. Further, the HR
estimate from the multivariate Cox analysis is more accurate and precise
compared to the stratified Cox analysis. Our findings point towards the
necessity of evaluating the impact of stratification factors on the time to
event analyses at the time of study design which is presently not a norm.
