title: Pandas new Column with list comprehension and an if statement with reference to existings columns
body: <p>I try to add a new column to a DataFrame with list comprehension and an if statement, this way:</p>
  
  <pre><code>SD['Ln(aT) ANALYTIC'] = [x + 1 for x in SD['T'] if SD['T'] &gt;= SD['TG']]
  </code></pre>
  
  <p>and I get this error:</p>
  
  <pre><code> The truth value of a Series is ambiguous. Use a.empty, 
   a.bool(), a.item(), a.any() or a.all().
  </code></pre>
  
  <p>I don´t know how to handle this problem.</p>
  
  <p>Any suggestions?</p>
  
  <p><strong>EDIT:</strong> DataFrame looks like:</p>
  
  <p><a href="https://i.stack.imgur.com/L6Bjq.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/L6Bjq.png" alt="enter image description here"></a></p>