title: Splitting a continuous variable into equal sized groups
body: <p>I need to split/divide up a continuous variable into 3 equal sized groups.</p>
  <p>Example data frame:</p>
  <pre><code>das &lt;- data.frame(anim = 1:15,
                    wt = c(181,179,180.5,201,201.5,245,246.4,
                           189.3,301,354,369,205,199,394,231.3))
  </code></pre>
  <p>After being cut up (according to the value of <code>wt</code>), I would need to have the 3 classes under the new variable <code>wt2</code> like this:</p>
  <pre><code>&gt; das 
     anim    wt wt2
  1     1 181.0   1
  2     2 179.0   1
  3     3 180.5   1
  4     4 201.0   2
  5     5 201.5   2
  6     6 245.0   2
  7     7 246.4   3
  8     8 189.3   1
  9     9 301.0   3
  10   10 354.0   3
  11   11 369.0   3
  12   12 205.0   2
  13   13 199.0   1
  14   14 394.0   3
  15   15 231.3   2
  </code></pre>
  <p>This would be applied to a large data set.</p>