Measuring (a Sufficient) World Model in LLMs: A Variance Decomposition Framework
This work addresses the need for semantic diagnostics to assess LLM reliability in high-stakes applications, but it is incremental as it builds on existing evaluation methods.
The authors tackled the problem of evaluating whether large language models (LLMs) possess a robust world model by proposing a variance decomposition framework to measure output consistency across semantically equivalent prompts, finding that larger models generally attribute more variability to user purpose changes, though improvements are modest and not uniform across domains.
Understanding whether large language models (LLMs) possess a world model-a structured understanding of the world that supports generalization beyond surface-level patterns-is central to assessing their reliability, especially in high-stakes applications. We propose a formal framework for evaluating whether an LLM exhibits a sufficiently robust world model, defined as producing consistent outputs across semantically equivalent prompts while distinguishing between prompts that express different intents. We introduce a new evaluation approach to measure this that decomposes model response variability into three components: variability due to user purpose, user articulation, and model instability. An LLM with a strong world model should attribute most of the variability in its responses to changes in foundational purpose rather than superficial changes in articulation. This approach allows us to quantify how much of a model's behavior is semantically grounded rather than driven by model instability or alternative wording. We apply this framework to evaluate LLMs across diverse domains. Our results show how larger models attribute a greater share of output variability to changes in user purpose, indicating a more robust world model. This improvement is not uniform, however: larger models do not consistently outperform smaller ones across all domains, and their advantage in robustness is often modest. These findings highlight the importance of moving beyond accuracy-based benchmarks toward semantic diagnostics that more directly assess the structure and stability of a model's internal understanding of the world.