n class="fu">.</span><span class="op">(</span><span class="va">mcid</span>, <span class="va">institution</span><span class="op">)</span><span class="op">]</span></span>
<span><span class="fu"><a href="https://rdrr.io/pkg/data.table/man/setkey.html" class="external-link">setkey</a></span><span class="op">(</span><span class="va">x</span>, <span class="va">mcid</span><span class="op">)</span></span>
<span><span class="va">x</span></span>
<span><span class="co">#&gt; Key: &lt;mcid&gt;</span></span>
<span><span