Subdomain 1.1: Core principles of supervised and unsupervised machine learning models.
1.An engineer compares a single decision tree to a random forest trained on the same fraud-detection dataset. The random forest achieves noticeably more stable performance across different random train/test splits than the single tree does. Which mechanism inside the random forest is primarily responsible for this stability?
- A.Each tree is trained on a bootstrap sample of the rows and a random subset of features at each split, and predictions are averaged or voted across trees to cancel out individual trees' errors
- B.The forest replaces the recursive splitting rule with a single global linear boundary that is less sensitive to any one training example
- C.Every tree in the forest is trained on the identical full dataset, and the forest simply keeps the tree that scores highest on the training set
- D.The forest applies principal component analysis to the feature set before training so that only the top components reach any individual tree
Show answer & explanation
Correct answer: A — Each tree is trained on a bootstrap sample of the rows and a random subset of features at each split, and predictions are averaged or voted across trees to cancel out individual trees' errors
- A. Bootstrap aggregating (bagging) combined with random feature subsampling at each split decorrelates the individual trees, and averaging or majority-voting their predictions reduces the variance that makes a single tree unstable across resamples.
- B. Random forests still use recursive axis-aligned splits in every tree; there is no global linear decision boundary replacing the tree structure, so this does not describe how the ensemble achieves stability.
- C. Training every tree on the identical dataset and keeping only the best performer would not reduce variance at all, since it discards the diversity that bagging depends on and risks selecting the tree that most overfits the training data.
- D. Random forests do not require a PCA preprocessing step; the randomness that stabilizes the ensemble comes from bootstrap row sampling and random feature selection at each split, not from projecting onto principal components.