Partial reinforcement is a psychological term for a type of
positive reinforcement where the reward is not granted every time the desired action is taken. It might be given
every n times the action is taken, or every n minutes if that action has been taken even once, or
randomly.
Animal studies have shown that partial reinforcement takes longer to train a behaviour
than constant positive reinforcement - but the animal will continue to perform the desired action for much longer without reinforcement. Of the various types of partial reinforcement, random reinforcement keeps 'em coming back for more without reward the longest. In one experiment, a pigeon given a random reward for pecking a button over a one minute interval continued pecking for over three hours without reward.
The effectiveness of random partial reinforcement is the cornerstone of casino gambling,
keeping people coming back for more for extended periods because they are only rewarded
at random intervals.
This effect may be the basis of many superstitions as well. If in a short interval, you randomly have good luck about every 10th time you throw salt over your shoulder, you'll continue to throw salt over your shoulder long after there is any recent correlation with luck.
To the extent that you think like a pigeon, of course. Unfortunately, most people seem to fall prey to the effect of partial reinforcement fairly readily.