Susan is on a variable-interval schedule of reinforcement.
Step-by-step explanation:
The mentioned example is of reinforcement variable-schedule in conditioning of operant behaviour by Skinner. It deals with strengthening and weakening of behaviour.
According to the theory, ‘schedule’ means the frequency of reinforcement that has been delivered and variable means inconsistent time slot and ‘interval’ indicates that the reinforcement is regulated by time.
So, it can be said that this reinforcement means that they are practiced at varying time intervals. This example justifies this theory.