How Much More Probable is "Much More Probable"? Verbal Expressions for
  Probability Updates
  Bayesian inference systems should be able to explain their reasoning to
users, translating from numerical to natural language. Previous empirical work
has investigated the correspondence between absolute probabilities and
linguistic phrases. This study extends that work to the correspondence between
changes in probabilities (updates) and relative probability phrases, such as
"much more likely" or "a little less likely." Subjects selected such phrases to
best describe numerical probability updates. We examined three hypotheses about
the correspondence, and found the most descriptively accurate of these three to
be that each such phrase corresponds to a fixed difference in probability
(rather than fixed ratio of probabilities or of odds). The empirically derived
phrase selection function uses eight phrases and achieved a 72% accuracy in
correspondence with the subjects' actual usage.
