variance reduction

Anyone can say 'ensemble of trees'. The real answer is about decorrelating those trees and why that, not just averaging, is what drops variance. Here is what makes the forest more than the sum of its trees.

CUPED can cut the sample size an A/B test needs by half without touching validity. The signal is explaining why subtracting a pre-experiment covariate reduces variance but never biases the estimate. Here is the answer.