The whole of the right side in Equation (5) represents the speaker's assumption about the listener: given word w the listener would infer referent r that is salient in a discourse and less ambiguously referred to by word w. If P (r) is uniform over referents and P (w|r) is constant across words and referents, this listener model reduces to 1 |w| .
frank and Goodman (2012)'s speaker model in (3) is a special case of our speaker model in (4) that assumes uniform discourse salience and constant cost.