title: Subsetting data in pandas with complex operations
body: <p>I have a dataset which looks like this:</p>
  
  <pre><code>year_week  selling_amount 
  201012     43 
  201112     44
  -1-1       56
  </code></pre>
  
  <p>Now there is an anomaly here. One simple way of checking is if <code>year_week</code> has any entry which has size less than 6. How can we do that in pandas ? </p>