Validation of Conformal Prediction in Cervical Atypia Classification

TL;DR

This study validates conformal prediction methods for cervical atypia classification, highlighting their limitations in aligning with human labels and their potential in identifying ambiguous or out-of-distribution cases.

Contribution

It provides a comprehensive validation of conformal prediction in medical diagnosis, emphasizing the importance of truthful prediction sets aligned with human expectations.

Findings

Conventional coverage metrics overestimate performance.

Current methods often produce misaligned prediction sets.

Conformal prediction can identify ambiguous and out-of-distribution data.

Abstract

Deep learning based cervical cancer classification can potentially increase access to screening in low-resource regions. However, deep learning models are often overconfident and do not reliably reflect diagnostic uncertainty. Moreover, they are typically optimized to generate maximum-likelihood predictions, which fail to convey uncertainty or ambiguity in their results. Such challenges can be addressed using conformal prediction, a model-agnostic framework for generating prediction sets that contain likely classes for trained deep-learning models. The size of these prediction sets indicates model uncertainty, contracting as model confidence increases. However, existing conformal prediction evaluation primarily focuses on whether the prediction set includes or covers the true class, often overlooking the presence of extraneous classes. We argue that prediction sets should be truthful and valuable to end users, ensuring that the listed likely classes align with human expectations rather than being overly relaxed and including false positives or unlikely classes. In this study, we comprehensively validate conformal prediction sets using expert annotation sets collected from multiple annotators. We evaluate three conformal prediction approaches applied to three deep-learning models trained for cervical atypia classification. Our expert annotation-based analysis reveals that conventional coverage-based evaluations overestimate performance and that current conformal prediction methods often produce prediction sets that are not well aligned with human labels. Additionally, we explore the capabilities of the conformal prediction methods in identifying ambiguous and out-of-distribution data.