Learning Interpretable Legal Case Retrieval via Knowledge-Guided Case Reformulation (2024)

Abstract: Legal case retrieval for sourcing similar cases is critical in upholding judicial fairness. Different from general web search, legal case retrieval involves processing lengthy, complex, and highly specialized legal documents. Existing methods in this domain often overlook the incorporation of legal expert knowledge, which is crucial for accurately understanding and modeling legal cases, leading to unsatisfactory retrieval performance. This paper introduces KELLER, a legal knowledge-guided case reformulation approach based on large language models (LLMs) for effective and interpretable legal case retrieval. By incorporating professional legal knowledge about crimes and law articles, we enable large language models to accurately reformulate the original legal case into concise sub-facts of crimes, which contain the essential information of the case. Extensive experiments on two legal case retrieval benchmarks demonstrate superior retrieval performance and robustness on complex legal case queries of KELLER over existing methods.

## introduction legal case retrieval is vital for legal experts to make informed decisions by thoroughly analyzing relevant precedents, which upholds justice and fairness @xcite . this practice is crucial in both common law and civil law systems globally @xcite . in civil law, although following past cases (known as "stare decisis") is not mandatory, judges are still highly advised to consider previous cases to improve the accuracy and trustworthiness of their judgments. in legal case retrieval, both the query and the document are structured legal cases, distinguishing the task from other information retrieval (ir) tasks. specifically, as shown in figure 1 , a legal case document comprises several sections, such as procedure after trial and investigation, it has been established that mu is the paternal uncle of the defendant yan. the two parties had developed conflicts over inheritance issues, and prior to the incident, they had a quarrel over a trivial matter. in a bid to vent personal spite, yan took advantage of mu's absence and set fire to mu's house... ## methodology in this section, we first introduce some basic concepts in legal case retrieval. then we delve into the three core parts of our keller, including legal knowledge-guided case reformulation, relevance modeling, and dual-level contrastive learning. ## conclusion in this paper, we introduce keller, a ranking model that effectively retrieves legal cases with 1260 high interpretability. keller structures legal documents into hierarchical texts using llms and determines relevance through a cross-matching module. our tests on two expert-annotated datasets validate its effectiveness. in the future, we will enhance keller by incorporating additional specialized knowledge and generative models to refine performance and produce language explanations.