Variable reward
Also said as Variable ratio schedule, Random reward, Intermittent reinforcement
Rewards that come after an unpredictable number of actions. It is the same pay on average as a fixed schedule, but you can never tell when the next one lands.
In Ferster and Skinner's work this schedule produced fast, steady responding that kept going long after rewards stopped, because a long dry run looks like bad luck. It is a real source of excitement in loot, and it is also what makes gambling hard to leave, so decide which you are building.
Say it
“Use a variable drop for bonus loot, but keep the main progress bar fixed, so nobody ever feels stuck. Never sell a chance at the reward for real money.”
Where you’ve seen it
Diablo drops are the classic game example: the next monster could be the one with the good axe. It costs you nothing but time there, whereas a paid slot charges for every pull.
Not to be confused with random reward with a floor
A pure variable schedule has no limit on how long you can go dry. A pity timer caps it, which keeps the surprise while removing the worst runs.