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Variable-interval reinforcement describes a schedule where reinforcement is tied to the passage of time, but the exact waiting period before a reward is earned varies around an average. Because the interval is not fixed, the time until reinforcement can be unpredictable, leading to responses that occur at varying times but tend to be steady rather than clustered. This matches the idea of dependence on time elapsed with variable wait times. In contrast, fixed-interval reinforcement uses a specific, constant time before reinforcement and often produces a noticeable pause after reinforcement; fixed ratio and variable ratio depend on the number of responses, not on time.