title: comparison operator in a column with pandas
body: <p>Sample dataframe :</p>
  <pre><code>R_no.    Score
  
  101       10
  102       15
  103       Nan
  104       Nan
  105       9
  </code></pre>
  <p>If the score is less than 15 return fail, if Nan then return Not attempted else pass.
  The output should get in another column of a dataframe.</p>
  <p>Required output:</p>
  <pre><code>R_no.    Score      result  
  101       10         fail
  102       15         pass    
  103       Nan        not attempted
  104       Nan        not attempted
  105       9          fail
  </code></pre>