r" href="#arg-fixed"></a></dt>
<dd><p>a formula for the fixed-effects part of the model, including the outcome.</p></dd>

<dt id="arg-random">random<a class="anchor" aria-label="anchor" href="#arg-random"></a></dt>
<dd><p>a formula for the random-effects part of the model. This should only contain
    the right-hand side part, e.g., <code>~ time | id</code>, where <code>time</code> is a variable,
    and <code>id</code> the grouping factor. When the symbol <code>||</code> is used in the definition
    of th