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# Is it possible for a slot machine to never pay?

25 July 2009

John,

Yes, there certainly is a RNG in each machine and these are part of the EPROM, but if payoffs were determined entirely by randomness then there would be possibilities where some machines would never pay off. This however would be very bad for the casino owners. Also, if the slot was based solely on a RNG to determine a win, how could they ever advertise that their machines pay off 94.3% of the time?

The only logical thing is that yes there is an RNG that determines the result on the virtual reel but there must also be a count of the number of coins collected by a given slot machine and the number of coins paid out. This is the only real way to determine the actual payout of the machine. Therefore, if the payout is lower than advertised, the computer program would need some way of bringing the payoff back into line with the advertised rate. This would not be difficult to program since there could easily be another virtual reel that is used when the payoff is lower. Still use the RNG but have a greater payoff on the virtual reel to bring the payoff back into line with the advertised rate.

Mike

Dear Mike,

You're right, there is the possibility that a machine would never pay off. Let's look at how likely that would be. Assume we have a machine with a 10% hit frequency. That means that 9 spins out of 10 are losers.

The probability that your next spin will be a loser is 0.9. The probability that your next two spins will be losers is 0.81 (0.9 squared). The probability that your next five spins will be losers is 0.59049, your next 10 0.35, your next 100 0.0000265, and so on.

There is the possibility that the machine would never pay off, but it is incredibly unlikely. In any case, you are correct that machines that never hit anything would be very bad for the casino.

Now, you asked how casinos can advertise that their machines pay off 94.3% of the time? I think you misunderstood what they're saying. A casino may say that a machine pays back 94.3%, which means that in the long run it pays back 94.3% of the money played through it. Individual players will win or lose, but if they play \$100,000 in total through the machine, the machine will have won about \$5,700 from them.

To answer your question, let me ask you one. How can the results from tossing a pair of dice yield the pyramid of craps if the results of each toss are random?

What's random on the dice toss is the result of the next toss. Still, we know that in the long run, 7 tosses out of 36 will be a seven, 6 out of 36 will be a six or eight, and so on.

The results of a spin on a slot machine are the same as the results of a toss of the dice. On the dice, we don't know the total that will be thrown next, but we can calculate the probability of throwing each possible total. On the slot, we don't know what symbols will land on the payline next, but we can calculate the probabilities of those symbols landing on the payline by looking at the distribution of symbols on the virtual reels. If we take into account how much each winning combination is worth and the cost to play, we can calculate the long-term payback.

The mathematical name for the process that takes place in a slot machine is Random Sampling with Replacement. The key point of this type of sampling is that the probability of achieving any particular result is the same on each observation.

The process you described of having some function in the machine override the results yielded by polling the RNG is called a "secondary decision" and is illegal in the United States. All gaming jurisdictions in the United States follow Nevada's regulation that the results of polling the RNG must be displayed to the player and the program can't ignore or change the result if the program doesn't like it for some reason.

Furthermore, the function isn't needed. Research Random Sampling with Replacement to see why it's not needed.

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