A schedule in which reinforcement is delivered contingent on the last of a number of responses. (a) Fixed ratio (FR) schedule-A reinforcement schedule in which a constant number of responses must occur prior to the reinforced response. For example, an FR 3 schedule indicates that each third response is reinforced. (b) Variable ratio (VR) schedule A schedule in which a variable number of responses must occur prior to the reinforced response. The number of responses usually varies around a specified average. For example, a VR 6 means that an average of one of six performances is reinforced.