 variable.</p></td>
    </tr>
    <tr>
      <th>idvar</th>
      <td><p>The name of the variable that serves as the ID variable for grouping observations. Must be in quotes</p></td>
    </tr>
    <tr>
      <th>data</th>
      <td><p>dataframe that contains all variables specified in the formulas- in long format.</p></td>
    </tr>
    <tr>
      <th>knots</th>
      <td><p>Specified locations for internal knots in the spline basis. Defaults to NULL, which corresponds to no internal knots.</p></td>
    </t