Fine-Grained Uncertainty Quantification via Collisions

TL;DR

This paper introduces a novel fine-grained uncertainty metric called the collision matrix, which measures class distinguishability in classification tasks and can be estimated using contrastive learning techniques.

Contribution

The paper proposes the collision matrix as a new uncertainty measure, along with methods to estimate it from labeled data using contrastive models and matrix recovery techniques.

Findings

The collision matrix effectively quantifies class confusion.

Contrastive models can estimate the collision matrix from data.

Estimated collision matrix improves class posterior probability estimation.

Abstract

We propose a new and intuitive metric for aleatoric uncertainty quantification (UQ), the prevalence of class collisions defined as the same input being observed in different classes. We use the rate of class collisions to define the collision matrix, a novel and uniquely fine-grained measure of uncertainty. For a classification problem involving $K$ classes, the $K\times K$ collision matrix $S$ measures the inherent difficulty in distinguishing between each pair of classes. We discuss several applications of the collision matrix, establish its fundamental mathematical properties, and show its relationship with existing UQ methods, including the Bayes error rate (BER). We also address the new problem of estimating the collision matrix using one-hot labeled data by proposing a series of innovative techniques to estimate $S$. First, we learn a pair-wise contrastive model which accepts two inputs and determines if they belong to the same class. We then show that this contrastive model (which is PAC learnable) can be used to estimate the row Gramian matrix of $S$, defined as $G=SS^T$. Finally, we show that under reasonable assumptions, $G$ can be used to uniquely recover $S$, a new result on non-negative matrices which could be of independent interest. With a method to estimate $S$ established, we demonstrate how this estimate of $S$, in conjunction with the contrastive model, can be used to estimate the posterior class probability distribution of any point. Experimental results are also presented to validate our methods of estimating the collision matrix and class posterior distributions on several datasets.