title: Use .corr to get the correlation between two columns
body: <p>I have the following pandas dataframe <code>Top15</code>:
      <img src="https://i.stack.imgur.com/qqIZL.png" alt="enter image description here"></p>
  
  <p>I create a column that estimates the number of citable documents per person:</p>
  
  <pre><code>Top15['PopEst'] = Top15['Energy Supply'] / Top15['Energy Supply per Capita']
  Top15['Citable docs per Capita'] = Top15['Citable documents'] / Top15['PopEst']
  </code></pre>
  
  <p>I want to know the correlation between the number of citable documents per capita and the energy supply per capita. So I use the <code>.corr()</code> method (Pearson's correlation):</p>
  
  <pre><code>data = Top15[['Citable docs per Capita','Energy Supply per Capita']]
  correlation = data.corr(method='pearson')
  </code></pre>
  
  <p>I want to return a single number, but the result is:
  <img src="https://i.stack.imgur.com/K8P18.png" alt="enter image description here"></p>