<span class='no'>pre_test</span> <span class='kw'>&lt;-</span> <span class='fu'>data.frame</span>(<span class='kw'>item1</span> <span class='kw'>=</span> <span class='fu'>c</span>(<span class='fl'>1</span>, <span class='fl'>0</span>, <span class='fl'>0</span>, <span class='fl'>1</span>, <span class='fl'>0</span>), <span class='kw'>item2</span> <span class='kw'>=</span> <span class='fu'>c</span>(<span class='fl'>1</span>, <span class='fl'>NA</span>, <span class='fl'>0</span>, <span class='fl'>1</span>, <span