The question addressed by this paper is whether disfluency resembles Inter-Move Interval, a measure of reaction time in conversation, in displaying effects of the overall difficulty of conducting a coherent conversation. Five sources of difficulty are considered as potential causes of disfluency: planning and producing an utterance, comprehending the prior utterance, performing a communicative task, order effects, and interpersonal factors. A multiple regression analysis on simple disfluencies in the HCRC Map Task Corpus shows that planning and production make the major independent contribution to predicting the rate of disfluencies, with interpersonal variables and position in dialogue also contributing significantly. Notably, comprehension variables did not affect either the total rate of disfluency or the rate of individual kinds of disfluencies.