Lectures 7 and 8 of Machine Learning Techniques open the aggregation part of the course. T7 sorts ways of combining hypotheses into uniform, linear, and any blending (stacking), shows with a few lines of algebra that uniform blending reduces variance, and then uses the bootstrap to create diverse g_t from the single dataset you have: that is bagging. T8 reinterprets the bootstrap as example weighting, then deliberately up-weights the examples the previous hypothesis got wrong so the next one is forced to differ, and votes with α_t = ln √((1−ε_t)/ε_t): that is AdaBoost. Practice with Fall 2024 HW6 Q4 and Q9, plus HW7's bootstrap and AdaBoost proofs and a 500-round AdaBoost-Stump experiment on madelon. There are no official solutions.
Lectures 9–11 of Machine Learning Techniques tie three models together with one thread: trees plus aggregation. T9 treats a decision tree as conditional aggregation and covers C&RT's binary branching, Gini and regression impurity, pruning, categorical features, and surrogate branches. T10 applies bagging to fully grown trees; add random subspaces and random projections and you get a random forest, with free OOB validation and permutation-based feature importance. T11 re-derives AdaBoost as steepest descent in function space on the exponential error, then swaps in squared error to get GBDT, which fits regressions to residuals. Practice with the impurity and gradient boosting proofs in Fall 2024 HW7. There are no official solutions.
The Techniques half of Fall 2024 has two homework sets and a final project, and all three PDFs are public. HW6 covers kernels, soft-margin SVM, and aggregation; its programming part uses LIBSVM on the 3-vs-7 subproblem of mnist.scale to count support vectors, compute margins, and run 128 validation rounds. HW7 covers bootstrap, impurity, AdaBoost, gradient boosting, and neural networks; its programming part is a 500-round AdaBoost-Stump on madelon. The final project is a fictional baseball league, HTMLB: predict home-team wins across two Kaggle stages and write an English report of at most seven pages that compares at least four methods. There are no official solutions. On 2026-09-30 both Kaggle pages returned 404 without login, so outside readers probably cannot get the HTMLB data and should reproduce the same splits on a public dataset instead.
HW4 Problem 3 quantifies why voting trees get more accurate in three steps: with p=0.6 the Hoeffding bound needs B≈691 independent trees to reach 10⁻⁶; correlation ρ between trees floors ensemble variance at ρσ²; and random forests' dense ensembling is compared with MoE's sparse routing.
LLM Council is a local Web App Andrej Karpathy built over a weekend. It sends one question to multiple LLMs simultaneously, has them anonymously peer-review each other, and then a Chairman model synthesizes a final answer. Positioned as a small tool for comparing models while studying — 99% vibe coded with no plans for long-term maintenance — but the architecture itself is a minimal ensemble LLM implementation worth studying.