On the applied use of progressive-ratio schedules of reinforcement.

Examination of responding under various schedule arrangements is a core component of many analyses of operant behavior. Much of the pioneering work in applied behavior analysis was bred from laboratory research involving the exposure of nonhuman subjects to a variety of schedule arrangements. For example, Reynolds (1961) described a multiple-schedule arrangement in which one component consisted of reinforcement for not responding for a specific period of time (i.e., differential reinforcement of other behavior [DRO]). Exposure to the DRO schedule resulted in generally low rates of responding. Since the initial publication of the basic DRO schedule arrangement, DRO procedures have been frequently employed in the treatment of destructive behavior (e.g., Cowdery, Iwata, & Pace, 1990; Mazaleski, Iwata, Vollmer, Zarcone, & Smith, 1993). Hodos (1961) described a schedule arrangement in which the requirement to access reinforcement increased on a trial-by-trial basis within the course of a single session. That is, the subject would initially emit a predetermined number of responses before reinforcement delivery (e.g., 20 responses). Following reinforcer delivery, the subsequent response requirement would increase by some increment (e.g., another 10 responses; referred to as a step size) such that in the next trial the subject would be required to complete more responses than in previous trials (e.g., 30 responses before reinforcer delivery, 40 responses before reinforcer delivery). Within-session changes in response requirements in this pattern constitute a progressive-ratio (PR) schedule of reinforcement. (It should be noted that increasing response requirements over successive sessions, e.g., DeLeon, Iwata, Goh, & Worsdell, 1997; Tustin, 1994; has been conceptualized as a form of PR schedule arrangement by some, e.g., Cooper, Heron, & Heward, 2007. For the purpose of the current discussion, PR schedules will be conceptualized as those schedules that increase during the course of a single session.) The primary clinical application of PR schedules involves assessment and quantification of differential reinforcer efficacy, which has sometimes been referred to as reinforcer potency (i.e., the ability of a reinforcer to maintain behavior; Baron, Mikorski, & Schlund, 1992; Hodos & Kalman, 1963; Johnson & Bickel, 2006). The assessment of reinforcer potency using PR schedules is commonly achieved through a comparison of relative break points (also referred to as breaking points). A break point is usually characterized as the last reinforced PR requirement that is completed. To illustrate, suppose a PR schedule were arranged with a step size of five responses. If a participant completed five trials and emitted 25 responses during the last trial before responding ceased with Stimulus A and completed eight trials and emitted 40 responses during the last trial with Stimulus B before responding ceased, one would conclude that Stimulus B was a more potent reinforcer than Stimulus A because Stimulus B had a higher break point. That is, Stimulus B supported more responding as the schedule requirements increased. In addition to the use of break points as a measure of reinforcer efficacy, PR schedules are also characterized by the omission of a terminal schedule value. That is, PR schedule requirements typically increase throughout the course of a session until responding stops for a period of time (e.g., 5 min) or until a predetermined duration of the observation has been reached (a session cap). Thus, total response output (as opposed to response rate) and break-point values are the primary measures of interest when evaluating behavior with PR schedules.