The distribution of interresponse times in the pigeon during variable-interval reinforcement.

Hart et al. (1968) watched six pigeons peck a key on a variable-interval (VI) schedule. Food came only after certain seconds passed, but the exact second changed randomly.

The team recorded every peck and drew pictures of the gaps between pecks, called inter-response times (IRTs). They wanted to see if VI schedules make those gaps follow a clear pattern.

Each bird showed a different IRT shape, and even the same bird changed shapes from day to day. The VI clock did not force a steady rhythm; timing stayed loose and messy.

In short, VI schedules give you variable response spacing—no tight control.

Gettinger (1993) later split each response into two parts: the pause right after food and the steady pecking that follows. That view helps explain why 1968 IRTs looked so scattered; pauses vary more than peck runs.

Guest et al. (2013) added a tiny, unsignaled delay between the peck and the grain. Short IRTs jumped up in the VI part but stayed flat in the VR part, showing that delay effects depend on schedule type.

Rogers-Warren et al. (1976) also tightened IRTs on purpose by making birds wait extra milliseconds before reinforcement. Their delay tactic worked, proving you can gain control if you add a timing rule beyond plain VI.

If you run VI schedules in a classroom or clinic, do not expect neat, stable response rates. Build in extra cues or brief delays if you need steadier timing, and always graph pauses separate from active responding.