Multi-Domain Ensembles for Domain Generalization

In this paper, we consider the challenging problem of multi-source zero shot domain generalization (MDG), where labeled training data from multiple source domains are available but with no access to data from the target domain. Many methods have been proposed to address this problem, but surprisingly the naiive solution of pooling all source data together and training a single ERM model is highly competitive. Constructing an ensemble of deep classifiers is a popular approach for building models that are calibrated under challenging distribution shifts. Hence, we propose MulDEns (Multi-Domain Deep Ensembles), a new approach for constructing deep ensembles in multi-domain problems that does not require to construct domain-specific models. Our empirical studies on multiple standard benchmarks show that MulDEns significantly outperforms ERM and existing ensembling solutions for MDG.