title: How does Weka calculate Precision using J48?
body: <p>I'm new to data mining and Weka. I built a classifier with <strong>J48</strong> in Weka using the GUI. When Weka is finished running it says:</p>
  
  <pre><code>Correctly Classified Instances 1035   -  68.543 %
  
  Incorrectly Classified Instances 475  -  31.457 %
  </code></pre>
  
  <p>When running from my own code (C# using IKVM.NET), I re-evaluate my model with the same unlabeled data set, save the result of the predicted class into an ARFF file and count the results I get from 1500 records.</p>
  
  <p>About 1300 records are correctly classified and 200 are not, which gives (I think) the <strong>precision</strong> measurement of 86%. </p>
  
  <p>Is this correct? Why does the results differ then?</p>