title: pandas data frame iterating over 2 index variables
body: <p>I have a data frame with 2 indexes called "DATE"( it is monthly data) and "ID" and a column variable named Volume. Now I want to iterate over it and fill for every unique ID a new column with the average value of the column Volume in a new column. </p>
  
  <p>The basic idea is to figure out which months are above the yearly avg for every ID. </p>
  
  <pre><code>list(df.index)
  
  (Timestamp('1970-09-30 00:00:00'), 12167.0)
  
  print(df.index.name)
  
  None
  </code></pre>
  
  <p>I seemed to not find a tutorial to address this :(</p>
  
  <p>Can someone please point me in the right direction </p>
  
  <pre><code>                    SHRCD  EXCHCD   SICCD     PRC     VOL       RET    SHROUT  \
  DATE       PERMNO                                                               
  1970-08-31 10559.0   10.0     1.0  5311.0  35.000  1692.0  0.030657   12048.0   
             12626.0   10.0     1.0  5411.0  46.250   926.0  0.088235    6624.0   
             12749.0   11.0     1.0  5331.0  45.500  5632.0  0.126173   34685.0   
             13100.0   11.0     1.0  5311.0  22.000  1759.0  0.171242   15107.0   
             13653.0   10.0     1.0  5311.0  13.125   141.0  0.220930    1337.0   
             13936.0   11.0     1.0  2331.0  11.500   270.0 -0.053061    3942.0   
             14322.0   11.0     1.0  5311.0  64.750  6934.0  0.024409  154187.0   
             16969.0   10.0     1.0  5311.0  42.875  1069.0  0.186851   13828.0   
             17072.0   10.0     1.0  5311.0  14.750   777.0  0.026087    5415.0   
             17304.0   10.0     1.0  5311.0  24.875  1939.0  0.058511    8150.0 
  </code></pre>