We are modeling data from a behavioral study in which subject pairs' conversations are coded for specific types of utterances (say "Type A"). Subjects decide when their trial is over and we count Type A utterances.
However, we disagree about whether we need an offset term for total amount of talk to model the amount of A. I understand that it's definitely necessary when causes external to subjects' behavior determine conversation length. But here, conversation length is influenced by subjects and how they were instructed (the type of instruction is a predictor).
So do we need an offset term here?