Has your machine really learned something? Snap quiz time

Machine learning (ML) is all about getting machines to learn but how do we know how well they are doing? Suppose we have existing data about people who buy books from our website – age, amount spent, preferred author and so on. These columns of data are known, in ML terms, as the "Predictors". For some of the customers we also know their gender, for others we don't, but we want to know it for all customers. We start with the customers where we do know the gender and we feed both the Predictor and Response data into a machine learning algorithm and get it to build a model that can (hopefully) forecast the Response from the Predictors.