The Size of Teachers as a Measure of Data Complexity: PAC-Bayes Excess Risk Bounds and Scaling Laws

We study the generalization properties of neural networks through the lens of data complexity. Recent work by Buzaglo et al. (2024) shows that random (nearly) interpolating networks generalize, provided there is a small "teacher" network that achieves small excess risk. We give a short single-sample PAC-Bayes proof of this result and an analogous "fast-rate" result for random samples from Gibbs posteriors. The resulting oracle inequality motivates a new notion of data complexity, based on the minimal size of a teacher network required to achieve any given level of excess risk. We show that polynomial data complexity gives rise to power laws connecting risk to the number of training samples, like in empirical neural scaling laws. By comparing the "scaling laws" resulting from our bounds to those observed in empirical studies, we provide evidence for lower bounds on the data complexity of standard benchmarks.