title: fit-transform on training data and transform on test data
body: <p>I am having trouble understanding how exactly <code>transform()</code> and <code>fit_transform()</code> are working together.</p>
  
  <p>I call <code>fit_transform()</code> on my training data set and <code>transform()</code> on my test set afterwards.</p>
  
  <p>However if I call <code>fit_transform()</code> on the test set I get bad results.</p>
  
  <p>Can anybody give me an explanation how and why this occurs?</p>