This masterclass, led by a presenter with Kaggle grandmaster level experience across more than 200 competitions, breaks down how to choose the right evaluation metric for a machine learning problem. It works through regression metrics (RMSE, MAE, MAPE, SMAPE, R squared, Pearson correlation, and RMSLE) and classification metrics (binary cross-entropy, accuracy, precision, recall, F1, and AUC), explaining the tradeoffs of each. Throughout, the framing is that metric choice is a business and risk question as much as a modeling one, using the same numeric error in different contexts to show why one metric can be right in one setting and wrong in another.