A Gentle Introduction to Conformal Prediction and Distribution-Free Uncertainty Quantification (2107.07511v6)

Abstract: Black-box machine learning models are now routinely used in high-risk settings, like medical diagnostics, which demand uncertainty quantification to avoid consequential model failures. Conformal prediction is a user-friendly paradigm for creating statistically rigorous uncertainty sets/intervals for the predictions of such models. Critically, the sets are valid in a distribution-free sense: they possess explicit, non-asymptotic guarantees even without distributional assumptions or model assumptions. One can use conformal prediction with any pre-trained model, such as a neural network, to produce sets that are guaranteed to contain the ground truth with a user-specified probability, such as 90%. It is easy-to-understand, easy-to-use, and general, applying naturally to problems arising in the fields of computer vision, natural language processing, deep reinforcement learning, and so on. This hands-on introduction is aimed to provide the reader a working understanding of conformal prediction and related distribution-free uncertainty quantification techniques with one self-contained document. We lead the reader through practical theory for and examples of conformal prediction and describe its extensions to complex machine learning tasks involving structured outputs, distribution shift, time-series, outliers, models that abstain, and more. Throughout, there are many explanatory illustrations, examples, and code samples in Python. With each code sample comes a Jupyter notebook implementing the method on a real-data example; the notebooks can be accessed and easily run using our codebase.

Overview of Conformal Prediction and Distribution-Free Uncertainty Quantification

The paper "A Gentle Introduction to Conformal Prediction and Distribution-Free Uncertainty Quantification" by Anastasios N. Angelopoulos and Stephen Bates provides a comprehensive examination of conformal prediction (CP) and its application in distribution-free uncertainty quantification (UQ). Conformal prediction is gaining traction for its ability to furnish prediction intervals with finite-sample guarantees, independent of the underlying data distribution or model assumptions.

Key Concepts and Methodologies

Conformal prediction offers a versatile framework for generating prediction sets applicable to diverse machine learning tasks, including classification and regression. The methodology underpinning CP involves:

1. Calibration: Utilizing a set of i.i.d. calibration data to create prediction sets with marginal coverage properties.
2. Nonconformity Scores: Computing these scores to gauge how atypical new observations might be relative to previously seen data.
3. Quantile Calculation: Determining threshold quantiles of calibration scoring to ensure appropriate level of coverage for prediction sets.

Practical Implementations

The paper outlines several implementations of conformal procedures to demonstrate versatility:

• Classification with Adaptive Prediction Sets (APS): Enhancing set calibration to avoid under/over-coverage among subgroups.
• Conformalized Quantile Regression: Leveraging quantile regression for continuous interval prediction with asymptotic properties of coverage.
• Conformalizing Bayesian Models: Utilizing posterior distributions to construct optimal prediction sets with Baysian risk currently under technical assumptions.

Evaluation and Extensions

It discusses methods to evaluate conformal prediction, emphasizing the importance of adaptivity—designing CP procedures that handle easy and hard examples differently.

In terms of theoretical extensions, the paper explores:

• Group-Balanced Conformal Prediction: Ensures consistent error rates across different demographic or categorical groups.
• Handling Distribution Shifts: Introducing techniques for weighted conformal prediction to manage covariate shifts and distribution drift, vital in scenarios of nonstationary data.

Theoretical and Practical Implications

The paper posits conformal prediction as a rigorous tool that bridges gaps in uncertainty quantification, providing:

• Theoretical Guarantees: CP's finite-sample guarantees address common pitfalls in statistical learning frameworks where assumption violations often lead to model failures.
• Practical Utility: The paper demonstrates applications from weather prediction to tumor segmentation, underscoring CP's adaptability across domains.

Future Directions

Future research directions suggested include refining score functions to enhance performance on structured prediction tasks and further investigating CP under challenging data paradigms such as high-dimensional settings or time series analysis.

Conclusion

This work underscores conformal prediction's potential in fostering reliable predictive systems, particularly in high-stakes environments. By providing comprehensive insights into both conceptual underpinnings and technical implementations, the authors make a noteworthy contribution to advancing the field of distribution-free uncertainty quantification in machine learning.