Enhancing Psychological Assessments with Open-Ended Questionnaires and Large Language Models: An ASD Case Study

Open-ended questionnaires allow respondents to express freely, capturing richer information than close-ended formats, but they are harder to analyze. Recent natural language processing advancements enable automatic assessment of open-ended responses, yet its use in psychological classification is underexplored. This study proposes a methodology using pre-trained large language models (LLMs) for automatic classification of open-ended questionnaires, applied to autism spectrum disorder (ASD) classification via parental reports. We compare multiple training strategies using transcribed responses from 51 parents (26 with typically developing children, 25 with ASD), exploring variations in model fine-tuning, input representation, and specificity. Subject-level predictions are derived by aggregating 12 individual question responses. Our best approach achieved 84% subject-wise accuracy and 1.0 ROC-AUC using an OpenAI embedding model, per-question training, including questions in the input, and combining the predictions with a voting system. In addition, a zero-shot evaluation using GPT-4o was conducted, yielding comparable results, underscoring the potential of both compact, local models and large out-of-the-box LLMs. To enhance transparency, we explored interpretability methods. Proprietary LLMs like GPT-4o offered no direct explanation, and OpenAI embedding models showed limited interpretability. However, locally deployable LLMs provided the highest interpretability. This highlights a trade-off between proprietary models' performance and local models' explainability. Our findings validate LLMs for automatically classifying open-ended questionnaires, offering a scalable, cost-effective complement for ASD assessment. These results suggest broader applicability for psychological analysis of other conditions, advancing LLM use in mental health research.