unary class text classification in weka?

Question

I have a training dataset (text) for a particular category (say Cancer). I want to train a SVM classifier for this class in weka. But when i try to do this by creating a folder 'cancer' and putting all those training files to that folder and when i run to code i get the following error: weka.classifiers.functions.SMO: Cannot handle unary class!

what I want to do is if the classifier finds a document related to 'cancer' it says the class name correctly and once i fed a non cancer document it should say something like 'unknown'.

What should I do to get this behavior?

Answer 1

The SMO algorithm in Weka only does binary classification between two classes. Sequential Minimal Optimization is a specific algorithm for solving an SVM and in Weka this a basic implementation of this algorithm. If you have some examples that are cancer and some that are not, then that would be binary, perhaps you haven't labeled them correctly.

However, if you are using training data which is all examples of cancer and you want it to tell you whether a future example fits the pattern or not, then you are attempting to do one-class SVM, aka outlier detection.

LibSVM in Weka can handle one-class svm. Unlike the Weka SMO implementation, LibSVM is a standalone program which has been interfaced into Weka and incorporates many different variants of SVM. This post on the Wekalist explains how to use LibSVM for this in Weka.