title: pandas dataframe (Selection)
body: <p>easy pandas dataframe problem here.</p>
  
  <p>I create dataframe by reading csv file, print it</p>
  
  <pre><code>    &lt;class 'pandas.core.frame.DataFrame'&gt;
      Int64Index: 176 entries, 0 to 175
      Data columns (total 8 columns):
      ID            176  non-null values
      study         176  non-null values
      center        176  non-null values
      initials      176  non-null values
      age           147  non-null values
      sex           133  non-null values
      lesion age    35  non-null values
      group         35  non-null values
      dtypes: float64(2), int64(1), object(5)
  </code></pre>
  
  <p>why this give me an error when I try to select row from dataframe according to certain 
  conditions</p>
  
  <pre><code>SUBJECTS[SUBJECTS.study=='NO2' and SUBJECTS.center=='Hermann']
  </code></pre>
  
  <p>Error Information:</p>
  
  <pre><code>    ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
  </code></pre>
  
  <p>Thank you very much in advance.</p>