h the survival probability should be estimated or <code>NULL</code> (default). If <code>NULL</code> the survival probability is estimated at all points in time at which an event occurred in the pooled sample.</p></dd>
<dt>bootstrap</dt>
<dd><p>A logical variable indicating whether bootstrapping should be performed or not. In bootstrapping, a number of simple random samples with replacement of size <code>nrow(data)</code> are drawn from <code>data</code>. For each sample the calculations are repeated and use