An overview of Federated Importance-Weighted Empirical Risk Minimization. Marginal train and test distributions of clients are arbitrarily different leading to intra-client and inter-client covariate shifts. To control privacy leakage, the server randomly shuffles unlabelled test samples and broadcasts to the clients.


Addressing Distribution Shifts in Federated Learning for Enhanced Generalization Performance

June 4, 2023

Federated learning is a powerful paradigm to collaboratively train a shared machine learning model among multiple clients, such as hospitals and cellphones, without sharing local data. Existing federated learning literature mainly focuses on training a model under the classical empirical risk minimization (ERM) paradigm, with implicitly assuming that the training and test data distributions of each client are the same. However, this stylized setup overlooks the specific requirements of each client.

Even for a single client, the distribution shift between training and test data, i.e., intra-client distribution shift,has been a major challenge for decades. For instance, scarce disease data for training and test in a local hospital can be different. We focus on the overall generalization performance on multiple clients  and modify the classical ERM to obtain an unbiased estimate of an overall true risk minimizer under intra-client and inter-client covariate shifts, develop an efficient density ratio estimation method under stringent privacy requirements of federated learning, and show importance-weighted ERM achieves smaller generalization error than classical ERM.




Federated Learning under Covariate Shifts with Generalization Guarantees

June 1, 2023

Ali Ramezani-Kebrya, Fanghui Liu, Thomas Pethick, Grigorios Chrysos, Volkan Cevher

Paper abstract

This paper addresses intra-client and inter-client covariate shifts in federated learning (FL) with a focus on the overall generalization performance. To handle covariate shifts, we formulate a new global model training paradigm and propose Federated Importance-Weighted Empirical Risk Minimization (FTW-ERM) along with improving density ratio matching methods without requiring perfect knowledge of the supremum over true ratios. We also propose the communication-efficient variant FITW-ERM with the same level of privacy guarantees as those of classical ERM in FL. We theoretically show that FTW-ERM achieves smaller generalization error than classical ERM under certain settings. Experimental results demonstrate the superiority of FTW-ERM over existing FL baselines in challenging imbalanced federated settings in terms of data distribution shifts across clients.