How do you handle class imbalance in training?

Handling class imbalance in training involves techniques to address situations where some classes in a dataset have significantly fewer examples than others. This imbalance can lead models to perform poorly on underrepresented classes, as they tend to prioritize accuracy by favoring majority classes. The goal is to ensure the model learns meaningful patterns from all classes, not just the most frequent ones. Common approaches include modifying the dataset, adjusting loss functions, and using specialized algorithms that account for imbalance.

One practical method is resampling the dataset. For undersampling, you reduce the number of examples in the majority class by randomly removing instances until class sizes are balanced. Oversampling does the opposite by duplicating or generating synthetic examples for the minority class, such as with SMOTE (Synthetic Minority Oversampling Technique). For example, in a fraud detection dataset where 95% of transactions are legitimate, oversampling the fraudulent cases (5%) can help the model recognize subtle fraud patterns. Another approach is using class weights in loss functions. Frameworks like PyTorch or TensorFlow allow assigning higher weights to minority classes during training. For instance, if a class has 10% of the data, its weight might be set to 10, forcing the model to penalize errors on that class more heavily. These adjustments guide the model to focus on underrepresented examples.

Advanced techniques include ensemble methods and anomaly detection. Algorithms like BalancedRandomForest or EasyEnsemble create multiple subsets of the data with balanced class distributions and combine their predictions. For extreme imbalances (e.g., 1:10,000), treating the problem as anomaly detection—where the minority class is treated as an outlier—can be effective. For example, in medical diagnosis for rare diseases, models like Isolation Forest or One-Class SVM might identify patterns in healthy patients and flag deviations. It’s also critical to use metrics like precision-recall curves, F1-score, or AUC-ROC instead of accuracy, as they better reflect performance on imbalanced data. Testing different combinations of these methods and validating with stratified cross-validation ensures robustness before deployment.