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# How random slot machines can pay back a given percent

8 December 2009

Slot machines are as random as humans can program a computer to be.

Slot machines also are programmed in a way that will lead toward a targeted payback percentage.

That mix — programmed, yet random — is something that has confused many a player in the decades since slots went electronic. Slot manufacturers have to be able to program payback percentages to comply with the law in states that set minimum and maximum returns. And the machines still have to be able to meet state randomness standards.

As one reader put it in a recent e-mail, "How is it that the casino is able to set its payback rate to within the specification set by state law? I don't seem to be able to get my head around the total separation of the random number generator from the play of the machine, that would permit the setting of the payback rates."

The confusion seems to be over exactly what it is that the programmer is programming.

It's not a matter of telling the game that it must pay a designated percentage. It's a matter of setting the possibilities and the odds of the game so that random results eventually will lead to the desired return.

In that way, slots are like table games. Take roulette. On an American double-zero wheel, the game is "programmed" with 38 possible results — the numbers 1 through 36 plus 0 and 00. The numbers come up randomly, and when you win on a single number, you're paid at 35-1 odds, a bit less than the true odds of 37-1. That gives the house an edge of 5.26%, or to turn it around, gives the game a programmed payback percentage of 94.74%.

There is nothing to keep your number from coming up two or three times in a row, and nothing that says it has to come up within several dozen spins or more. But given enough trials, the random results and the odds of the game will lead to something very close to roulette's "programmed percentage."

Same deal in craps. By using two six-sided dice, we "program" the game with 36 possible outcomes — one way to make 2, two 3s, three 4s, four 5s, five 6s, six 7s, five 8s, four 9s, three 10s, two 11s and one 12. The odds of a given roll combined with how much the casino pays for a winner yield a "programmed" house edge, or payback percentage.

The probability of rolling any given number remains constant, but it is possible to roll the same number several times in a row. When you do, it doesn't make it any more or less likely that you'll roll it again next time. The odds just remain the same, and in the long run any hot or cold streak will just fade into statistical insignificance.

Slots work more or less the same way, except that there are hundreds or thousands of possibilities instead of 38. For regular play on the reels, randomly occurring numbers are programmed, each corresponding to a reel symbol. To make up an example, the programmer might write it so that every time the random number 1 shows up, the reel shows a jackpot symbol; with Nos. 2, 3 or 4, it shows a 7, with Nos. 5-9, a triple bar, and so on. The possibilities are programmed, but when they turn up is random, just like it's random when a 7 turns up in craps.

Same deal in bonus rounds. The possibilities are programmed, and then a random number generator determines which possibilities you see on the reels or screen. In the case of a pick'em-type bonus round, whether you're picking gift boxes, fish or alien ships for bonuses, the size of your bonus is not predetermined and your choices make a difference.

If you happen to do well and win a larger-than-average bonus, the machine doesn't go into makeup mode. Over a long period of time, normal results according to the odds of the game will yield a normal payback percentage, and your big win fades into statistical insignificance.

Just as when a table games designer sets the rules of a card, dice or wheel game, the slot programmer sets the possible outcomes, and the pay table gives you back a little less than the true odds of hitting the winners. You can hit several winners in a row, or none for a number of spins. Results are random, but eventually they will lead to something very close to the programmed payback percentage.

Programmed, yes. Random, yes. Just like any other casino game, but in an electronic sort of way.

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John Grochowski

John Grochowski is the best-selling author of The Craps Answer Book, The Slot Machine Answer Book and The Video Poker Answer Book. His weekly column is syndicated to newspapers and Web sites, and he contributes to many of the major magazines and newspapers in the gaming field, including Midwest Gaming and Travel, Slot Manager, Casino Journal, Strictly Slots and Casino Player.

Listen to John Grochowski's "Casino Answer Man" tips Tuesday through Friday at 5:18 p.m. on WLS-AM (890) in Chicago. Look for John Grochowski on Facebook and Twitter @GrochowskiJ.