How to measure a classifier's quality. You've just written a new classification algorithm and want to measure how well it performs on a test set, and compare it with other classifiers. What performance measure should you use? There are several standard answers. Let's assume the classifier gives an output y(x), where x is the input, which we won't discuss further, and that the true target value is t. In the simplest discussions of classifiers, both y and t are binary variables, but you might care to consider cases where y and t are more general objects also.