title: Pandas - Add mean, max, min as columns in dataframe
body: <p>I have a <code>df =</code></p>
  <pre><code>         statistics  s_values
  year
  1999  cigarette use       100
  1999  cellphone use       310
  1999   internet use       101
  1999    alcohol use       100
  1999       soda use       215
  2000  cigarette use       315
  2000  cellphone use       317
  2000   internet use       325
  2000    alcohol use       108
  2000       soda use       200
  2001  cigarette use       122
  2001  cellphone use       311
  2001   internet use       112
  2001    alcohol use       144
  2001       soda use       689
  </code></pre>
  <p>I calculated the max, min and mean based on the <code>year</code> <code>index</code> and <code>statistics</code> <code>column</code>.</p>
  <p>I want to insert the mean, max and min as columns in the data frame where the output result looks like this</p>
  <p>my desired output:</p>
  <pre><code>         statistics   s_values        mean  min     max
  year                    
  1999    alcohol use     100.0        104.0  100.0   108.0
  1999    cellphone use   310.0        313.5  310.0   317.0
  1999    cigarette use   100.0        207.5  100.0   315.0
  1999    internet use    101.0        213.0  101.0   325.0
  1999    soda use        215.0        207.5  200.0   215.0
  2000    alcohol use     108.0        104.0  100.0   108.0
  2000    cellphone use   317.0        313.5  310.0   317.0
  2000    cigarette use   315.0        207.5  100.0   315.0
  2000    internet use    325.0        213.0  101.0   325.0
  2000    soda use        200.0        207.5  200.0   215.0
  2001    alcohol use     144.0        104.0  100.0   108.0
  2001    cellphone use   311.0        313.5  310.0   317.0
  2001    cigarette use   122.0        207.5  100.0   315.0
  2001    internet use    112.0        213.0  101.0   325.0
  2001    soda use        689.0        207.5  200.0   215.0   
  </code></pre>
  <p>I tried to do the following but values in the columns  are all <code>NaN</code></p>
  <pre><code>gen_mean = df.groupby('statistics').mean()
  gen_min = df.groupby('statistics').min()
  gen_max = df.groupby('statistics').max()
  
  df.insert(2, 'Gen Avg', gen_mean)
  df.insert(3, 'Gen Max', gen_max)
  df.insert(4, 'Gen Min', gen_min)
  </code></pre>
  <p>Thank you</p>