title: How to get the frequency of NaN obsevations in a pandas dataframe column
body: <p>I have a pandas data frame with 83 columns and 4000 rows.  I intend to use the data for a logistic regression and therefore want to narrow down my columns to those that have the least amount of missing data.  </p>
  
  <p>To do this I was thinking of ranking them based on the frequency of NaN observations. I tried a few things like </p>
  
  <pre><code>econ_balance["BG.GSR.NFSV.GD.ZS"].describe()
  econ_balance["BG.GSR.NFSV.GD.ZS"].value_counts
  econ_balance["BG.GSR.NFSV.GD.ZS"]["NaN"]
  econ_balance["BG.GSR.NFSV.GD.ZS"][NaN]
  </code></pre>
  
  <p>None of which seem to work. I always tried googling to see if this question has been answered before but no luck.</p>
  
  <p>Thanks in advance for the help</p>
  
  <p>Josh</p>