Heterogeneous Treatment Effect with Trained Kernels of the Nadaraya-Watson Regression
This addresses causal inference in observational studies with imbalanced data, offering a novel approach but likely incremental as it builds on established regression techniques.
The paper tackles estimating conditional average treatment effects when control samples are abundant but treatment samples are scarce, proposing TNW-CATE, a method that trains kernels in Nadaraya-Watson regression using a neural network, and shows it outperforms existing learners like T-learner and X-learner in simulations.
A new method for estimating the conditional average treatment effect is proposed in the paper. It is called TNW-CATE (the Trainable Nadaraya-Watson regression for CATE) and based on the assumption that the number of controls is rather large whereas the number of treatments is small. TNW-CATE uses the Nadaraya-Watson regression for predicting outcomes of patients from the control and treatment groups. The main idea behind TNW-CATE is to train kernels of the Nadaraya-Watson regression by using a weight sharing neural network of a specific form. The network is trained on controls, and it replaces standard kernels with a set of neural subnetworks with shared parameters such that every subnetwork implements the trainable kernel, but the whole network implements the Nadaraya-Watson estimator. The network memorizes how the feature vectors are located in the feature space. The proposed approach is similar to the transfer learning when domains of source and target data are similar, but tasks are different. Various numerical simulation experiments illustrate TNW-CATE and compare it with the well-known T-learner, S-learner and X-learner for several types of the control and treatment outcome functions. The code of proposed algorithms implementing TNW-CATE is available in https://github.com/Stasychbr/TNW-CATE.