From Chapter One in Evaluating Machine Learning Models by Alice Zheng:
One of the core tasks in building a machine learning model is to evaluate its performance. It’s fundamental, and it’s also really hard. My mentors in machine learning research taught me to ask these questions at the outset of any project: “How can I measure success for this project?” and “How would I know when I’ve succeeded?” These questions allow me to set my goals realistically, so that I know when to stop. Sometimes they prevent me from working on ill-formulated projects where good measurement is vague or infeasible. It’s important to think about evaluation up front.