his string in the model formula.</p></td>
    </tr>
    <tr>
      <th>k</th>
      <td><p>Choice of knots (for the smoothing function <code>s</code>); the
default is 4 to avoid over-parameterization.</p></td>
    </tr>
    <tr>
      <th>a</th>
      <td><p>The lower quantile value of the selected threshold variable, which
the estimated threshold is not allowed to exceed; the default is 0.2.</p></td>
    </tr>
    <tr>
      <th>b</th>
      <td><p>The upper quantile value of the selected threshold variabl