A speaker often decides whether or not to saysomething based on his assessment of the impact itwould have on his hearer's beliefs. If he thinks itwould bring them more in line with the truth, he saysit; otherwise he does not. In this paper, I developa model of these judgments, focusing specifically onthose of vague sentences. Under the simplifyingassumption that an utterance only conveys a speaker'sapplicability judgments, I present a Bayesian model ofan utterance's impact on a hearer's beliefs. Fromthis model I (...) derive a model of a speaker's judgment ofwhether or not an utterance would be informative. Iillustrate it with several examples of judgments ofvague and non-vague sentences. For instance, I showthat it models the common judgment that assertingeither ``George is tall'' or ``George is not tall'' wouldbe misleading if George were borderline tall, butasserting ``George is tall and he isn't tall'' would notbe. (shrink)