Responses per unit time. Several types have been distinguished: overall or average rate, determined over a substantial time such as an experimental session; local, momentary, or moment-to-moment rate, determined over a short time, particularly when it is relatively constant throughout that time; running rate, roughly equivalent to local rate, but sometimes with the provision that it is determined over a time bounded by pauses; and terminal rate, determined over a short time just before a reinforcer, especially in an FI. Criteria for distinguishing segments of performance, such as the response sequence over which a running rate is determined, may be based on informal criteria such as visual inspection, but they can be defined more explicitly (e.g., a period of no responding may be treated as a pause only if it is more than 5 s long). -- Other terms distinguish changes in rate. acceleration or positive acceleration is a gradual increase, appearing as concave-upward curvature on a cumulative record; deceleration or negative acceleration is a gradual decrease, appearing as concave-downward curvature; cyclic changes are repeated increases and decreases, each completed over a roughly constant time; and compensation is a low rate immediately following an unusually high one, or a high one immediately following an unusually low one. The acceleration typically produced by an FI schedule is often called a scallop, especially in reference to its appearance in a cumulative record. The term has been extended to accelerations produced by other schedules but is ordinarily restricted to accelerations bounded by some event, such as a reinforcer. The curvature in an FI has been measured in terms of quarter-life (the time to complete one quarter of the responses within an interval) and index of curvature (a statistic based on the number of responses in successive fractions of an interval). Moment-to-moment changes in rate are often described in terms of grain (e.g., a relatively constant rate is said to have a finer grain than one that rapidly fluctuates), again, especially in reference to the appearance of a cumulative record.