Fraud detection often has few labeled fraud cases but many unlabeled transactions. Semi-supervised learning uses both: a small labeled set guides learning from a much larger, mostly legitimate unlabeled pool. This can reduce reliance on expensive manual labeling and expose the model to more data patterns. section 7.4.3
Two common approaches differ in how they use unlabeled transactions:
- Self-training: train on labeled data, predict labels for unlabeled transactions, add high-confidence predictions to the labeled set, then retrain. section 7.4.3
- Co-training: train multiple models, often on different feature subsets, and use their confident or agreeing predictions to label unlabeled instances. section 7.4.3




