title: python pandas - build subset based on column names
body: <p>Here is my multi-index dataframe:</p>
  
  <pre><code>                             A                    B
                               Net   Upper   Lower  Mid  Zsore
  Answer option                                                
  More than once a day          0%   0.22%  -0.12%   2    65 
  Once a day                    0%   0.32%  -0.19%   3    45
  Several times a week          2%   2.45%   1.10%   4    78
  Once a week                   1%   1.63%  -0.40%   6    65
  </code></pre>
  
  <p>I would like to build a subset by COLUMN NAME on the second level (nothing fancy) which keeps the following columns only "Net, Upper, Zscore" </p>
  
  <pre><code>                             Net   Upper  Zsore
  Answer option                                               
  More than once a day          0%   0.22%    65 
  Once a day                    0%   0.32%    45
  Several times a week          2%   2.45%    78
  Once a week                   1%   1.63%    65
  </code></pre>
  
  <p>My failed attempt to pull this off, I guess it fails because I don't know how to account for the 2 levels of columns?</p>
  
  <pre><code>df = df[[u'Net',u'Upper',u'Zsore']]
  </code></pre>