Beyond Size and Class Balance: Alpha as a New Dataset Quality Metric for Deep Learning

In deep learning, achieving high performance on image classification tasks requires diverse training sets. However, dataset diversity is incompletely understood. The current best practice is to try to maximize dataset size and class balance. Yet large, class-balanced datasets are not guaranteed to be diverse: images can still be arbitrarily similar. We hypothesized that, for a given model architecture, better model performance can be achieved by maximizing dataset diversity more directly. This could open a path for performance improvement without additional computational resources or architectural advances. To test this hypothesis, we introduce a comprehensive framework of diversity measures, developed in ecology, that generalizes familiar quantities like Shannon entropy by accounting for similarities and differences among images. (Dataset size and class balance emerge from this framework as special cases.) By analyzing thousands of subsets from seven medical datasets representing ultrasound, X-ray, CT, and pathology images, we found that the best correlates of performance were not size or class balance but $A$ -- ``big alpha'' -- a set of generalized entropy measures interpreted as the effective number of image-class pairs in the dataset, after accounting for similarities among images. One of these, $A_0$, explained 67\% of the variance in balanced accuracy across all subsets, vs. 54\% for class balance and just 39\% for size. The best pair was size and $A_1$ (79\%), which outperformed size and class balance (74\%). $A$ performed best for subsets from individual datasets as well as across datasets, supporting the generality of these results. We propose maximizing $A$ as a potential new way to improve the performance of deep learning in medical imaging.