title: Obtaining threshold values from a ROC curve
body: <p>I have some models, using <code>ROCR</code> package on a vector of the predicted class percentages, I have a performance object. Plotting the performance object with the specifications "tpr", "fpr" gives me a ROC curve.</p>
  
  <p>I'm comparing models at certain thresholds of false positive rate (x).  I'm hoping to get the value of the true positive rate (y) out of the performance object. Even more, I would like to get the class percentage threshold that was used to generate that point.</p>
  
  <p>the index number of the false positive rate (<code>x-value</code>) that is closest to the threshold without being above it, should give me the index number of the appropriate true positive rate (<code>y-value</code>). I'm not exactly sure how to get that index value.</p>
  
  <p>And more to the point, how do i get the threshold of class probabilities that was used to make that point?</p>