title: python pandas add a lower level column to multi_index dataframe
body: <p>Could someone help me to achieve this task?
  I have data in multi-level data frame through the unstack() operation:</p>
  
  <pre><code>Original df:
  Density  Length  Range  Count
    15k    0.60  small    555
    15k    0.60    big     17
    15k    1.80  small    141
    15k    1.80    big     21
    15k    3.60  small    150
    15k    3.60    big     26
    20k    0.60  small   5543
    20k    0.60    big     22
    20k    1.80  small    553
    20k    1.80    big     25
    20k    3.60  small    422
    20k    3.60    big     35
  
  df  = df.set_index(['Density','Length','Range']).unstack('Range')
  
  # After unstack:
                    Count       
  Range             big  small
  Density Length              
  15k     0.60       17    555
          1.80       21    141
          3.60       26    150
  20k     0.60       22   5543
          1.80       25    553
          3.60       35    422
  </code></pre>
  
  <p>Now I try to add an extra column in level 1. it is the ratio of the small/big. I have tried the following syntax, no error but with different outcomes</p>
  
  <pre><code>#df[:]['ratio']=df['Count']['small']/df['Count']['big'] ## case 1. no error, no ratio
  #df['Count']['ratio']=df['Count']['small']/df['Count']['big'] ## case 2. no error, no ratio
  #df['ratio']=df['Count']['small']/df['Count']['big'] ## case 3. no error, ratio on column level 0
  df['ratio']=df.ix[:,1]/df.ix[:,0]                    ## case 4. no error, ratio on column level 0
  
  #After execution above code, df:
                    Count         ratio
  Range             big  small       
  Density Length                     
  15k     0.60       17    555  32.65
          1.80       21    141   6.71
          3.60       26    150   5.77
  20k     0.60       22   5543 251.95
          1.80       25    553  22.12
          3.60       35    422  12.06
  </code></pre>
  
  <p>I don't understand why case 1 &amp; 2 show no error neither adding new ratio column. and why in case 3 &amp; 4 the ratio column is on level 0, not the expected level 1. Also like to know if there is a better/concise way to achieve this. Case 4 is the best I can do but I don't like the implicit indexing way (instead of using the name) to refer to a column.   </p>
  
  <p>Thanks</p>