Aschedule in operant conditioning rewards behaviors an average number of times on an unpredictable basis.

A variant-ratio schedule rewards behaviors an average number of times on an unpredictable basis. The schedule has to be a ratio because it refers to a number of times the behavior must occur to be rewarded not the amount of time that must pass like an interval schedule. The schedule cannot be fixed because it says "average" not a specific amount of time or number of behaviors; therefore, it is a variant-ratio schedule.