title: pandas dataframe delete rows with low frequency
body: <p>What is the best practice to remove all rows that has a column with low frequency value?</p>
  
  <p>Dataframe:</p>
  
  <pre><code>IN:
  foo bar poo
  1   a   A
  2   a   A
  3   a   B
  4   b   B
  5   b   A
  6   b   A
  7   c   C
  8   d   B
  9   e   B
  </code></pre>
  
  <p>Example 1:
  Remove all rows that have less than 3 in frequency value in column 'poo':</p>
  
  <pre><code>OUT:
  foo bar poo
  1   a   A
  2   a   A
  3   a   B
  4   b   B
  5   b   A
  6   b   A
  8   d   B
  9   e   B
  </code></pre>
  
  <p>Example 2:
  Remove all rows that have less than 3 in frequency value in column 'bar':</p>
  
  <pre><code>OUT:
  foo bar poo
  1   a   A
  2   a   A
  3   a   B
  4   b   B
  5   b   A
  6   b   A
  </code></pre>