Joshua is interested in running a half-marathon and seeks help from a personal trainer. During the first week of training, Joshua is asked to walk 5 miles every day. The following week, the trainer instructs him to walk a mile and then run a mile until he completes 5 miles. Each week, the training regimen gets harder. By the end of the month, Joshua is running 5 miles without stopping. By the end of the second month, Joshua is running the distance of a half-marathon (approx. 13 miles). After each training session, the trainer meets with Joshua to give him reinforcing feedback and to motivate him to continue training. This situation is an example of:

The case shown in the question above can be considered an example of a variable-ratio schedule, which is a type of reinforcement scheme in operant conditioning, where a response is observed in the elaboration of an unpredictable number of responses stimulated through repetitive training, or that is, this type of reinforcement scheme encourages the performance of a specific and desired behavior, through various "trainings" with variable responses, until the desired behavior is achieved.

In the case above, the desired behavior was Joshua's ability to run the equivalent of a half marathon. This behavior was stimulated with several trainings that allowed Joshua to run different amounts of kilometers, until he could run the desired amount.