Implicit Target Shift in Online Learning: Characterization and Correction

Abstract

Online learning from a stream of data is a defining feature of intelligence, yet modern machine learning systems often struggle in this setting, especially under distributional shift. To understand its basic properties, we study the relationship between online and offline learning in the context of kernel regression by deriving a closed-form expression for the function learned by online kernel regression. We reveal that online kernel regression is equivalent to offline regression with shifted, inaccurate target outputs. Conversely, we show that by compensating for this implicit target shift in the teaching signal through target correction, online kernel-based learning can provably learn the same predictor as its offline counterpart. We derive both a closed-form expression for this target correction and an iterative form that can be applied sequentially. Applying this framework to continual image classification tasks on domain-incremental Split CIFAR-10 and CORe50, we show that online stochastic gradient descent with iteratively corrected targets outperforms learning with the true targets. This work therefore provides a basic framework for analyzing and improving online learning in non-stationary environments from the perspective of implicit target shift.