Keep tomorrow out of today

Target leakage happens when a model uses information that would not have been available at the time of prediction, or information too close to the answer it is supposed to predict. It is like letting a student see the answer key before the test.

PPE includes a feature gate that checks for several forms of leakage, including future data and downstream effects. It also uses overfitting guards. These are important because a beautiful score can come from an unfair test.

The safe question is: “Could this exact signal have been known when the field team had to act?” If not, the signal belongs in analysis after the event, not in a forecast before it.