title: why we use training data in weka evaluation function?
body: <p>I am using weka for classification. I am using different train and test dataset. I noticed that while evaluating on test dataset, we use training data in evaluation function. Does anyone know why we use training data? why not test data? I mean in following code, why we use trains in line 6 ? why not tests?</p>
  
  <pre><code>        1.  trainsource = new DataSource(train_file_path);
          2.  trains = trainsource.getDataSet();
          3.  trains.setClassIndex(0);
  
          4.  testsource = new DataSource(test_file_path);
          5.  tests = testsource.getDataSet();
  
  
          6.  evaluation= new Evaluation(**trains**);     
          7. model.buildClassifier(trains);
          8. evaluation.evaluateModel(model, tests);
  </code></pre>
  
  <p>Thanks! in advance!!</p>