Enhancing Thematic Analysis with Local LLMs: A Scientific Evaluation of Prompt Engineering Techniques

Thematic Analysis (TA) is a powerful tool for human factors, HCI, and UX researchers to gather system usability insights from qualitative data like open-ended survey questions. However, TA is both time consuming and difficult, requiring researchers to review and compare hundreds, thousands, or even millions of pieces of text. Recently, this has driven many to explore using Large Language Models (LLMs) to support such an analysis. However, LLMs have their own processing limitations and usability challenges when implementing them reliably as part of a research process – especially when working with a large corpus of data that exceeds LLM context windows. These challenges are compounded when using locally hosted LLMs, which may be necessary to analyze sensitive and/or proprietary data. However, little human factors research has rigorously examined how various prompt engineering techniques can augment an LLM to overcome these limitations and improve usability. Accordingly, in the present paper, we investigate the impact of several prompt engineering techniques on the quality of LLM-mediated TA. Using a local LLM (Llama 3.1 8b) to ensure data privacy, we developed four LLM variants with progressively complex prompt engineering techniques and used them to extract themes from user feedback regarding the usability of a novel knowledge management system prototype. The LLM variants were as follows:1.A “baseline” variant with no prompt engineering or scalability2.A “naïve batch processing” variant that sequentially analyzed small batches of the user feedback to generate a single list of themes3.An “advanced batch processing” variant that built upon the naïve variant by adding prompt engineering techniques (e.g., chain-of-thought prompting)4.A “cognition-inspired” variant that incorporated advanced prompt engineering techniques and kept a working memory-like log of themes and their frequencyContrary to conventional approaches to studying LLMs, which largely rely upon descriptive statistics (e.g., % improvement), we systematically applied a set of evaluation methods from behavioral science and human factors. We performed three stages of evaluation of the outputs of each LLM variant: we compared the LLM outputs to our team’s original TA, we had human factors professionals (N = 4) rate the quality and usefulness of the outputs, and we compared the Inter-Rater Reliability (IRR) of other human factors professionals (N = 2) attempting to code the original data with the outputs generated by each variant. Results demonstrate that even small, locally deployed LLMs can produce high-quality TA when guided by appropriate prompts. While the “baseline” variant performed surprisingly well for small datasets, we found that the other, scalable methods were dependent upon advanced prompt engineering techniques to be successful. Only our novel "cognition-inspired" approach performed as well as the “baseline” variant in qualitative and quantitative comparisons of ratings and coding IRR. This research provides practical guidance for human factors researchers looking to integrate LLMs into their qualitative analysis workflows, disentangling and uncovering the importance of context window limitations, batch processing strategies, and advanced prompt engineering techniques. The findings suggest that local LLMs can serve as valuable and scalable tools in thematic analysis.