title: Standard deviation on group returns null
body: <p>Pandas beginner here. I would like to calculate the standard deviation of <code>SPEED</code> for ships that depart on the same date.</p>
  
  <p>I've tried the following and it returns <code>NaN</code> - why is that? The correct answer should be <code>9.9500418759588</code></p>
  
  <pre><code>import pandas as pd
  df = pd.DataFrame({ 'LEFT PORT DATE':['21/10/2019','21/10/2019','21/10/2019','20/10/2019'], 'SPEED':[10, 20, 0.10, 50]})
  
  df['RUN STD DEVIATION']  =  df.groupby('LEFT PORT DATE')['SPEED'].std()
  </code></pre>