pan class="va">student</span><span class="op">$</span><span class="va">institution</span><span class="op">)</span><span class="op">)</span></span>
<span><span class="co">#&gt; [1] "Institution B" "Institution C" "Institution J"</span></span></code></pre></div>
<p>Race/ethnicity and sex are often used as grouping variables.</p>
<div class="sourceCode" id="cb5"><pre class="downlit sourceCode r">
<code class="sourceCode R"><span><span class="co"># Possible values</span></span>
<span><span class="fu"><a href="h