title: how does sklearn jaccard_score gets calculated?
body: <p>I was trying to understand what is going on with sklearn's jaccard_score.</p>
  
  <p>This is the result I got</p>
  
  <pre><code>1. jaccard_score([0 1 1], [1 1 1])
  0.6666666666666666
  
  2. jaccard_score([1 1 0], [1 0 0])
  0.5
  
  3. jaccard_score([1 1 0], [1 0 1])
  0.3333333333333333
  </code></pre>
  
  <p>I understand that the formula is </p>
  
  <pre><code>intersection / size of A + size of B - intersection
  </code></pre>
  
  <p>I thought the last one should give me 0.2 because the overlap is 1 and total number of entries is 6 resulting 1/5. but I got 0.33333...</p>
  
  <p>Can anyone explain how sklearn calculates jaccard_score?</p>