title: problem on calculating mean a column in Python
body: <p>I used Python to calculate a column average in a dataframe. I used the following command to calculate  the summation of data in defined column. But, I do not exactly know why the output result is just appended data without any summation.</p>
  
  <pre><code>df3 = pd.merge(df1, df2, how = 'inner', on = 'key')
  df3
  df3['age'].sum()
  </code></pre>
  
  <p>The output result is:</p>
  
  <pre><code>&gt;&gt;key      data1        data2   age
  
  42           0.01         1200  17
  
  101          0.02          523  10
  
  &gt;&gt;1710
  </code></pre>
  
  <p>So, as you see the sum function is just append '17' and '10' and the result is 1710. For mean function I again get 1710/2 = 855.</p>
  
  <p>Could you please put any comment?</p>