wdata"></a></dt>
<dd><p>A data frame with two rows, each specifying a set of covariate values.
The hazard ratio is calculated as hazard(z2)/hazard(z1), where z1 is the first row
of <code>newdata</code> and z2 is the second row.</p>
<p><code>newdata</code> must be supplied unless the model <code>x</code> includes just one covariate.
With one covariate, a default is constructed, which defines the hazard ratio between
the second and first level of the factor (if the covariate is a factor), or between
a value o