Fine-tuned Large Language Models Can Replicate Expert Coding Better than Trained Coders: A Study on Informative Signals Sent by Interest Groups

Understanding the political process in the United States requires examining how information is provided to politicians and the general public. While existing studies point to interest groups as strategic information providers, studying this aspect empirically has been challenging due to the need for expert-level annotation in measurement. We make two contributions. First, we demonstrate that fine-tuned large language models (LLMs) can replicate expert-level annotation in a specialized area above the accuracy of lightly-trained workers, crowd-workers, and zero-shot LLMs. Second, we quantify two types of interest group signals that are difficult to separate empirically using other means: 1) informative signals that help agents improve political decisions, and 2) associative signals that influence preference formation but lack direct relevance to the substantive topic of interest. We demonstrate the utility of this approach using two applications where our classifier generalizes out of distribution. This study shows methodologically the applicability of large language models for complex expert-driven measurement tasks but also shows substantively that interest groups strategically tailor the composition of signals under different institutional settings.