Regina, to get a simple ROC curve, use the following sequence of commands: pred <- prediction(predictions, labels) perf <- performance(pred, "tpr", "fpr") plot(perf) In the first line, 'predictions' are the raw predictions (usually numerical) of your classifier, and labels (as you correctly guessed) the true (binary) classes of your items. The true positive and false positive rates _at various cutoffs_ are then calculated from the raw predictions. The purpose of ROCR is to obtain these (and other) rates --- if you already have them, I don't understand from your email what else you want.
Just in case you are uncertain about the overall framework of classification, take a look at this tutorial: Fawcett, T. (2003): ROC graphs: Notes and practical considerations for data mining researchers http://www.hpl.hp.com/techreports/2003/HPL-2003-4.pdf The following slide deck also contains a brief introduction of the framework, as well as usage examples of ROCR: http://rocr.bioinf.mpi-sb.mpg.de/ROCR_Talk_Tobias_Sing.ppt Hope that helps, Tobias On Thu, Mar 19, 2009 at 3:01 AM, Regina Beretta Mazaro <rberet...@hotmail.com> wrote: > > > > > Hi, > > I'm involved in a bioinformatics project at my university, and we're doing a > comparison paper between some methods of classification of nc-RNA. I've been > encharged of ploting the ROC curves' graphs. But I'm new on working with R > and I'm having some difficulty with the prediction-class. I don't get where > the values of ROCR.simple$predictions, for example, came from ($labels I > understand that represents the real classisfication of that item). And I just > have the values for true positive, false positive, true negative and false > positive, obtained from the methods tests. So, I can't plot a graph with my > own values. How can I convert these values that I have into $predictions-type > needed to run ROCR? Is there any function that does this? Or I have to redo > the tests using another kind of measuring? If someone could help me, I'll be > very grateful. > Regina Beretta Mazaro. > _________________________________________________________________ > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
