The child gets his potato chips every Saturday for doing his chores for the week.
fixed interval and variable ratio.
In this situation, the child receives potato chips every Saturday for completing his chores for the week. This means that the interval between receiving potato chips is fixed, as he always receives them on Saturdays. However, the ratio of potato chips to chores is not fixed, as the child may do more or fewer chores from one week to the next and receive a corresponding amount of potato chips. Therefore, the appropriate schedules of reinforcement in this situation are fixed interval and variable ratio.
When they change it to Friday, and then the chips still remain in an unknown amount, its a Variable Interval with a Variable Ratio.