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# A weird question about the slot machine's RNG

30 August 2010

John,

On a previous question you had said that the outcome of a spin polls the RNG for the next spin. The way I understand this is that future spins are a result of previous spins. Did I understand correctly? If this is true does that mean that if (this question gets weird) on two separate time lines the same machine could have different results even if the button were pressed at the exact same time?

Also I have used your estimating payback method on several machines and have found it doesn't work in my case. I used the results of three machines and found payback using probability of a symbol landing on the line gave me significantly higher payback amounts than the actual payback. Are you certain that each reel is random and not the complete combination? It would appear that the machines are programmed to deceive you into thinking you were close to winning big. I know I've missed the big one by one symbol thousands of times (that's right, thousands). How could this be if the third jackpot symbol lands one in 24 spins?

Kris

Dear Kris,

All of the RNG algorithms I've seen (and I don't claim to have seen all of them) use one or more prior results in calculating the next result. The RNG algorithm in a slot machine is more correctly called a psuedo-RNG -- the number stream from the RNG satisfies many requirements for randomness, but they're not truly random.

The answer to your question about two different results is no. If you could somehow travel back in time to press the Spin button at the exact same time, you would get the same result. Each time you press the button, the machine is in the exact same state. You will get the same result.

As for estimating payback, each reel is random, thus the combination is also random. I'm not surprised that your calculated payback is not near the payback you experienced. It takes hundreds of thousands, if not millions, of spins for the payback you experience to home in on the machine's long-term payback. That's why tracking results is not a practical way to estimate a machine's long-term payback. I don't know how many spins you tracked and therefore how close your estimate is to the actual, but your calculated payback should be much closer to the machine's actual long-term payback than the payback you experienced over those spins.

Finally, to paraphrase Jessica Rabbit, slot machines aren't programmed to show near misses, they're just drawn that way. The programming running the slot machine doesn't decide to show a near miss. The frequency with which near misses occur comes as a result of the layout of the symbols on the virtual reels. Slot designers can tinker with the virtual reel layout to make near misses occur more or less frequently.

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