title: P-Value to test hypothesis shows NaN
body: <p>I am getting NaN for p.value when trying to test the null hypothesis that the mean of the revenue for the phone plan surf is the same as that of the ultimate plan. I don't understand what I am doing wrong. I'm assuming that it may have to do with my DataFrame <code>call_plan_merge</code>. There are some NaN values in the <code>monthly_revenue</code> column (not visible in what I posted here). Could that be the reason why? But at the same time the calculated mean (which we can see was calculated properly while ignoring the NaNs from the <code>monthly_revenue</code> column) is already in the variables used for testing the hypothesis, so I don't understand NaN would be generated for p-value.</p>
  <p>Here is my code:</p>
  <pre><code>
  #The average revenue from users of Ultimate and Surf calling plans differs.
  average_rev_surf = call_plan_merge.query('tariff == &quot;surf&quot;')
  average_rev_surf = average_rev_surf['monthly_revenue'].mean()
  
  average_rev_ultimate = call_plan_merge.query('tariff == &quot;ultimate&quot;')
  average_rev_ultimate = average_rev_ultimate['monthly_revenue'].mean()
  
  alpha = 0.05  # critical statistical significance
  
  results = st.ttest_1samp(average_rev_surf, average_rev_ultimate)
  
  print('p-value:', results.pvalue)
  
  if results.pvalue &lt; alpha:
      print('We reject the null hypothesis')
  else:
      print(&quot;We can't reject the null hypothesis&quot;) 
      
  print('Average revenue for the surf plan is: {:.2f}$'.format(average_rev_surf))  
  print('Average revenue for the ultimate plan is: {:.2f}$'.format(average_rev_ultimate))
  </code></pre>
  <p>Output:</p>
  <pre><code>p-value: nan
  We can't reject the null hypothesis
  Average revenue for the surf plan is: 35.77$
  Average revenue for the ultimate plan is: 36.32$
  </code></pre>
  <p>This is what <code>call_plan_merge</code> looks like:</p>
  <pre><code>    user_id  call_month  total_calls  duration    tariff  reg_month  churn_month state  monthly_revenue  
  0    1000.0        12.0         16.0     124.0  ultimate         12         13.0    GA            70.00  
  1    1001.0         8.0         27.0     182.0      surf          8         13.0    WA            20.00  
  2    1001.0         9.0         49.0     315.0      surf          8         13.0    WA            20.00  
  3    1001.0        10.0         65.0     393.0      surf          8         13.0    WA            90.09  
  4    1001.0        11.0         64.0     426.0      surf          8         13.0    WA            60.00  
  5    1001.0        12.0         56.0     412.0      surf          8         13.0    WA            60.00  
  6    1002.0        10.0         11.0      59.0      surf         10         13.0    NV            20.00  
  7    1002.0        11.0         55.0     386.0      surf         10         13.0    NV            60.00  
  8    1002.0        12.0         47.0     384.0      surf         10         13.0    NV            20.00  
  9    1003.0        12.0        149.0    1104.0      surf          1         13.0    OK           158.12  
  </code></pre>
  <p>Thank you so much for your help!</p>