title: DataFrame.drop not dropping expected rows in Pandas
body: <p>I have a Pandas DataFrame that includes rows that I want to drop based on values in a column "population":</p>
  
  <pre><code>data['population'].value_counts()
  
  general population                          21
  developmental delay                         20
  sibling                                      2
  general population + developmental delay     1
  dtype: int64
  </code></pre>
  
  <p>here, I want to drop the two rows that have <code>sibling</code> as the value. So, I believe the following should do the trick:</p>
  
  <pre><code>data = data.drop(data.population=='sibling', axis=0)
  </code></pre>
  
  <p>It does drop 2 rows, as you can see in the resulting value counts, but they were <em>not</em> the rows with the specified value.</p>
  
  <pre><code>data.population.value_counts()
  
  developmental delay                         20
  general population                          19
  sibling                                      2
  general population + developmental delay     1
  dtype: int64
  </code></pre>
  
  <p>Any idea what is going on here?</p>