Boosting combines weak learners into a stronger predictive model by training them sequentially. Each round emphasizes examples misclassified earlier, then combines predictions using a weighted vote for classification or weighted sum for regression. section 7.5 section 7.5.2
AdaBoost operationalizes the idea through an adaptive sequence:
- Initialize all training-example weights equally and fit the first weak classifier. section 7.5.2
- Increase weights for misclassified examples and decrease weights for correctly classified examples in later rounds. section 7.5.2
- Measure each learner’s weighted error and assign more influence to more accurate learners. section 7.5.2
Decision stumps provide a concrete base learner: each depth-one tree partitions feature space with a threshold. Gradient tree boosting extends the sequential strategy to differentiable loss functions for classification and regression. section 7.5.2 section 7.5.2
AdaBoost on feature data
A Python implementation trains depth-one decision trees on two Iris features, compares ensembles with 1, 2, 3, and 10 learners, and can also illustrate the method on an XOR dataset. Learning curves and decision regions show how accuracy changes as learners are added.


