Binary classifiers are accompanying us on a daily basis. Tests that detect disease, give us the answer: positive/negative, spam filters say spam/not spam, smartphones that authenticate us based on a face scan or fingerprint – make a known/unknown decision. The question: how to evaluate the efficiency of such a classifier does not seem extremely complicated. Just choose the one that will predict the most cases correctly. As many of us have already realized – the actual evaluation of a binary classifier requires somewhat more sophisticated means. But we’ll talk about that in a moment.