Schematic timeline depicting the bout-and-pause model of free-operant performance. A trial starts at the left-end of the timeline and progresses to the right. The time between trial onset and the first response (vertical line labeled "Response 1") is the latency for that trial. The time between two consecutive responses is an inter-response time, or IRT. There are two types of IRTs: within-bout (short) and between-bout (long). Thicker vertical lines are bout-initiation responses; thinner lines are within-bout responses. The length of a bout is the number of responses between latency and the first between-bout IRT, or between two between-bout IRTs.