title: How to calculate 1st and 3rd quartiles?
body: <p>I have DataFrame:</p>
  
  <pre><code>    time_diff   avg_trips
  0   0.450000    1.0
  1   0.483333    1.0
  2   0.500000    1.0
  3   0.516667    1.0
  4   0.533333    2.0
  </code></pre>
  
  <p>I want to get 1st quartile, 3rd quartile and median for the column <code>time_diff</code>. To obtain median, I use <code>np.median(df["time_diff"].values)</code>.</p>
  
  <p>How can I calculate quartiles?</p>