nd <code>data</code>).</p></td>
    </tr>
    <tr>
      <th>X</th>
      <td><p>A numeric matrix of covariates. If not supplied, an intercept-only
model will be fit. This is optional with the <code>chats</code> argument.</p></td>
    </tr>
    <tr>
      <th>initial_est</th>
      <td><p>(Optional) A vector of length 1 + ncol(X) giving the starting values for the likelihood maximisation search. The first element is the starting estimate for sigma^2_u, and the remaining elements are the starting elements fo