Bootstrap‐based methods for estimating standard errors in Cox's regression analyses of clustered event times

We propose two bootstrap‐based methods to correct the standard errors (SEs) from Cox's model for within‐cluster correlation of right‐censored event times. The cluster‐bootstrap method resamples, with replacement, only the clusters, whereas the two‐step bootstrap method resamples (i) the clusters, and (ii) individuals within each selected cluster, with replacement. In simulations, we evaluate both methods and compare them with the existing robust...