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# How can slots be random and pay back a set percentage?

21 November 2009

Sir,

Love your articles and read them all. My question is, and I know you have heard it before, but how can the RNG be what it is and yet the casino knows exactly what the payback is, 97.8, 98.6, 95.5, etc. over a certain period of time? How can something be random all through its play and still know what to hold for the casino's percentage? I have a hard time understanding how this could be.

Also, I just bought an IGT Game King Poker machine and would like to know where on the chip board or in the machine is the RNG 'thang' located and what it looks like?

Keep up the good work.

Thanks,
Bob

Dear Bob,

Thanks for the kind words about my columns.

You're not alone in having a hard time understanding how outcomes on slot machines can be random and how machines can hold a given percentage. I don't mind answering the question multiple times because I use a different approach each time and maybe one of them will make sense to someone who is confused.

The key thing is that the RNG is used to draw an outcome from a pool of outcomes and we know how many times each combination appears in that pool.

Consider this analogy. I have a bag filled with red, white and blue cards. Five cards are red, three are white and two are blue. You draw a card at random, note its color and then put the card back in the bag. Repeat.

In the long run, you'll draw a red card about half the time, a white card about one-third of the time, and a blue card about one-fifth of the time.

We don't know what color will be drawn on any given draw, but we know what the color distribution of a large number of draws will look like.

The same situation exists on the slot machine. We don't know what the outcome of the next spin will be, but we do know what the profile of a large number of outcomes will look like because we know how many times each outcome appears in the pool of possible outcomes. (Well, we don't know, but the slot's manufacturer and the casino knows.)

We know how frequently each outcome will occur in the long run and we know how much each combination is worth. That's all we need to know to calculate a machine's long-term payback. As the machine gets more and more play, the distribution of outcomes gets closer and closer to the distribution dictated by the number of times each symbol appears on each reel and the machine's operational payback gets closer and closer to its long-term payback. Really, the only thing we don't know is what the outcome of any spin will be.

The RNG is a mathematical function in the slot machine's programming. It will be on the program EPROM on the logic board. Discrete RNG chips are available, but I don't think any slots machines use them because the manufacturers use their own, proprietary functions.

Best of luck in and out of the casinos,
John

Send your slot and video poker questions to John Robison, Slot Expert, at slotexpert@comcast.net. Because of the volume of mail I receive, I regret that I can't reply to every question.

John Robison

John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

#### Books by John Robison:

The Slot Expert's Guide to Playing Slots
John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

#### Books by John Robison:

The Slot Expert's Guide to Playing Slots