an><span class="op">)</span>
<span class="co">#&gt; 'data.frame':    51020 obs. of  10 variables:</span>
<span class="co">#&gt;  $ id     : num  1 2 3 4 5 6 7 8 9 10 ...</span>
<span class="co">#&gt;  $ happy  : Factor w/ 3 levels "not too happy",..: 1 1 2 1 2 2 1 1 2 2 ...</span>
<span class="co">#&gt;  $ year   : num  1972 1972 1972 1972 1972 ...</span>
<span class="co">#&gt;  $ age    : num  23 70 48 27 61 26 28 27 21 30 ...</span>
<span class="co">#&gt;  $ sex    : Factor w/ 2 levels "male","female": 2 