Josiah checks his email several times throughout the day. some days there is mail each time he checks; sometimes several days go by with no new messages arriving. in this example, josiah's behavior of checking his email is being reinforced on:

In this example, Josiah's behavior of checking his electronic mail is being reinforced on "a variable-interval schedule".

A variable interval schedule (VI) is a sort of operant conditioning support plan for which reinforcement is given to a reaction after particular measure of time has passed (a flighty measure of time), yet this measure of time is on a changing/variable schedule.