 data.table</span></span>
<span><span class="va">data</span> <span class="op">&lt;-</span> <span class="fu">as.data.table</span><span class="op">(</span><span class="va">source_data</span><span class="op">)</span></span>
<span></span>
<span><span class="co"># set the data.table key for better indexing</span></span>
<span><span class="fu">setkey</span><span class="op">(</span><span class="va">data</span>, <span class="va">subjid</span>, <span class="va">param</span>, <span class="va">agedays</span><span clas