Most models that fail in production don't fail because of the algorithm. They fail because the features used the future, the test was a random split of the past, or the world changed after training. In this course you build a churn model for Paystream, a mobile wallet, from 65,825 raw transactions: define churn at a snapshot, engineer recency, frequency, value, trend and failure features as of each month-end, and see a leaked feature push the AUC to 0.98 and a random split flatter the model. You'll validate on later months with labels that were really known in time, compare gradient boosting with logistic regression, check calibration, turn lift into a costed calling plan, and watch a competitor's launch drift the model before retraining can catch up.