gender, <span class="dt">levels =</span> <span class="kw">c</span>(<span class="st">&quot;M&quot;</span>, <span class="st">&quot;F&quot;</span>))

<span class="co"># replace out-of-range data with missing values (NA) ---------</span>
t0<span class="op">$</span>age[t0<span class="op">$</span>age <span class="op">&gt;</span><span class="st"> </span><span class="dv">100</span>] &lt;-<span class="st"> </span><span class="ot">NA</span>
t0<span class="op">$</span>age[t0<span class="op">$</span>age <span class="op