title: Pandas: Finding mean value in multi-level index
body: <p>I have the following code just multi-indexing some ag data:</p>
  
  <pre><code>df_corn_mean = df_corn.set_index(['Year','Value'])
  df_corn_mean
  </code></pre>
  
  <p>It results in this:</p>
  
  <pre><code>                County        Commodity
  Year    Value       
  2016    171.2   BARTHOLOMEW   CORN
          180.1   BOONE         CORN
          190.6   CLINTON       CORN
          178.9   DECATUR       CORN
          179.5   HAMILTON      CORN
  </code></pre>
  
  <p>There are several more years.  I want the mean of the values per year.  I've tried to groupby, which I thought was the right approach, but I'm having trouble.  Any help is appreciated.</p>