title: whether pandas.decribe() excludes missing value rows?
body: <p><code>pandas.describe()</code> function generate descriptive statistics that summarize the dataset, <strong>excluding NaN values</strong>. But does the exclusion here means that the total count (i.e., rows of a variable) vary or fixed?</p>
  
  <p>For example, I calculate the <code>mean</code> by using <code>describe()</code> for a df with missing values:</p>
  
  <pre><code>varA
   1
   1
   1
   1
   NaN
  </code></pre>
  
  <p>Is the mean = 4/5 or 4/4 here?</p>
  
  <p>And how does it apply to other results in <code>describe</code>? For example, the standard deviation, quartiles?</p>
  
  <p>Thanks!</p>