an> <span class="op">&lt;-</span> <span class="va">Exam</span>              
<span class="co"># set standLRT to NA for 3 observations</span>
<span class="va">Exam2</span><span class="op">[</span><span class="fu"><a href="https://rdrr.io/r/base/c.html">c</a></span><span class="op">(</span><span class="fl">1</span>,<span class="fl">2</span>,<span class="fl">5</span><span class="op">)</span>,<span class="fl">7</span><span class="op">]</span> <span class="op">&lt;-</span> <span class="cn">NA</span>    
<span cl