 (row), the following metadata vectors are
required (or can be stored in a Data.Frame <code>metadata</code>):</p>
<ul>
<li>
<code>time</code>, the time at which the observation has been
taken.</li>
<li>
<code>ind</code> identifying which subject (individual) is
associated with the observation.</li>
</ul>
<p>Optionally:</p>
<ul>
<li>
<code>group</code> an identifier indicating to which study group the
observation belongs.</li>
</ul>
<blockquote>
<p>All observations of a given individual need to be affected t