title: cannot perform reduce with flexible type error in pandas
body: <p>I  have a dataset of 4 attributes like:</p>
  
  <pre><code>  taxi id   date time   longitude   latitude
  0   1   2/2/2008 15:36  116.51  39.92
  1   1   2/2/2008 15:46  116.51  39.93
  2   1   2/2/2008 15:56  116.51  39.91
  3   1   2/2/2008 16:06  116.47  39.91
  4   1   2/2/2008 16:16  116.47  39.92
  </code></pre>
  
  <p>datatype of each attribute is as follows:</p>
  
  <pre><code>taxi id    dtype('int64')
  date time  dtype('O')
  longitude   dtype('float64')
  latitude    dtype('float64')
  </code></pre>
  
  <p>i want to calculate mean and standard deviation (std) for each attribute.</p>
  
  <p>For mean i have tried this code:</p>
  
  <pre><code>np.mean('longitude')
  </code></pre>
  
  <p>but it gives me error like:</p>
  
  <pre><code>TypeError: cannot perform reduce with flexible type
  </code></pre>