What About the Precedent: An Information-Theoretic Analysis of Common Law

TL;DR

This study uses an information-theoretic approach to analyze how precedent influences case outcomes in common law, comparing the importance of arguments versus facts across European Court of Human Rights cases.

Contribution

It is the first computational analysis contrasting two jurisprudential theories on precedent influence using a large legal corpus and information theory.

Findings

Precedent arguments share 0.38 nats of information with case outcomes.

Precedent facts share 0.18 nats of information with case outcomes.

Arguments are generally more influential than facts in determining outcomes.

Abstract

In common law, the outcome of a new case is determined mostly by precedent cases, rather than by existing statutes. However, how exactly does the precedent influence the outcome of a new case? Answering this question is crucial for guaranteeing fair and consistent judicial decision-making. We are the first to approach this question computationally by comparing two longstanding jurisprudential views; Halsbury's, who believes that the arguments of the precedent are the main determinant of the outcome, and Goodhart's, who believes that what matters most is the precedent's facts. We base our study on the corpus of legal cases from the European Court of Human Rights (ECtHR), which allows us to access not only the case itself, but also cases cited in the judges' arguments (i.e. the precedent cases). Taking an information-theoretic view, and modelling the question as a case outcome classification task, we find that the precedent's arguments share 0.38 nats of information with the case's outcome, whereas precedent's facts only share 0.18 nats of information (i.e., 58% less); suggesting Halsbury's view may be more accurate in this specific court. We found however in a qualitative analysis that there are specific statues where Goodhart's view dominates, and present some evidence these are the ones where the legal concept at hand is less straightforward.