To Believe or Not to Believe Your LLM
Authors: name: Yasin Abbasi Yadkori, name: Ilja Kuzborskij, name: András György, name: Csaba Szepesvári

We explore uncertainty quantification in large language models (LLMs), with
the goal to identify when uncertainty in responses given a query is large. We
simultaneously consider both epistemic and aleatoric uncertainties, where the
former comes from the lack of knowledge about the ground truth (such as about
facts or the language), and the latter comes from irreducible randomness (such
as multiple possible answers). In particular, we derive an
information-theoretic metric that allows to reliably detect when only epistemic
uncertainty is large, in which case the output of the model is unreliable. This
condition can be computed based solely on the output of the model obtained
simply by some special iterative prompting based on the previous responses.
Such quantification, for instance, allows to detect hallucinations (cases when
epistemic uncertainty is high) in both single- and multi-answer responses. This
is in contrast to many standard uncertainty quantification strategies (such as
thresholding the log-likelihood of a response) where hallucinations in the
multi-answer case cannot be detected. We conduct a series of experiments which
demonstrate the advantage of our formulation. Further, our investigations shed
some light on how the probabilities assigned to a given output by an LLM can be
amplified by iterative prompting, which might be of independent interest.