Lucy is training her dog to sit by giving him treats. On average, she gives her dog a treat every three times he sits, but the exact number of times he needs to sit in order to receive a treat varies based on the trial. In other words, some days she might give her dog a treat after sitting twice, and on another day she might give her dog a treat after sitting four times. What kind of reinforcement schedule is this describing?

Fixed ratio

Variable interval

Variable ratio

Fixed interval