Variable interval reinforcement

Variable interval reinforcement (VI) schedules deliver reinforcement for the first response after a random average length of time passes since the last reinforcement
 * Example: VI3" = reinforcement is provided for the first response after an average of 3 seconds since the last reinforcement.
 * Lab example: VI10" = a rat is reinforced for the first bar press after an average of 10 seconds passes since the last reinforcement
 * Real world example: a predator can expect to come across a prey on a variable interval schedule