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The Slot Expert's Guide to Winning at Slots and Video Poker

17 February 2003

By John Robison

Frank Scoblete calls it the "god in the machine" in Break the One-Armed Bandits. Players know that it determines the symbols that land on the payline on a slot machine and the cards they're dealt on a video poker machine. It's the Random Number Generator or RNG in the machine. Let's take a close look at what the RNG is and how slot and video poker machines use it.

The RNG can either be a separate computer chip on one of the boards in the machine or it can just be part of the program that runs the slot machine. Either way, the RNG is nothing more than a series of mathematical steps performed on one number to generate another number. Those steps can be quite involved, so I won't go into any examples here. I'll just let this sample output from an RNG suffice. Suppose we start with the number 7 and feed it into our RNG. The RNG returns a 2. We feed the RNG the 2 and we get back an 8. We feed in the 8 and get a 9. We continue doing this and we end up with this series of numbers: 7, 2, 8, 9, 3, 6, 5, 1, 4, 7, 2, 8, etc. Once the RNG returns a 7, the pattern repeats.

Now, we have to take a quick digression and clean up some of our terminology. First, what is a random number? Is 13 a random number? How about 2,145,298?

Randomness is not a property of a number. Numbers can be odd, even, integer, non-integer, etc., but random is not a property of a single number. A sequence of numbers, however, can be random. Random in this context means that knowing one number in the sequence does not help you predict what the next number in the sequence will be.

You may recall from the RNG example I used before that we used the last number output from the RNG to calculate the next number. If we knew the formula used in the RNG and the last number produced, we'd be able to calculate the next number. That certainly doesn't fit the definition of random I just gave.

The RNG in a machine is more correctly called a Psuedo-Random Number Generator. The number sequence from a Psuedo-RNG satisfies many of the tests for randomness that we can perform, but the sequence is not truly random because given the current number, we can calculate the next number in the sequence.

So, we have this RNG function in our machine. In the early days of computer-controlled slot machines, the machines worked quite differently from today's machines. The computers were not as powerful as those used today, so the machine would figure out the outcome of the next spin as soon as the current spin was finished. And the machine would sit there waiting for someone to play it so it could reveal the outcome that it had already determined.

Sitting with an outcome locked in made it easy for RNG cheats. They were able to figure out where the RNG was in the sequence, so they would know what the outcome of any spin would be. Then they could bet full coin on the winning spins and one coin on the losers.

The computers in today's machines are much more powerful and the RNG is constantly running, generating new outcomes. The outcome you see is the number produced by the RNG at the split second you push the spin button or pull the handle. How quickly does the RNG run? It depends on the machine. An RNG can generate from hundreds to thousands of outcomes each second.

Now, what does the machine do with the output from the RNG? Let's look at slot machines first. The machine has to somehow take a number and map it into the symbols to display on the payline. Here's one way a machine can do this.

Let's say our machine has five reels with 100 symbols on each reel. This is either a Big (BIG) Bertha or a video machine. We'll make our solution real easy and get a separate number for each reel. The player hits the spin button and we check the RNG for the last number it generated. The number might be 2,347,174, but we only need numbers from 1 to 100, so we ignore everything but the last two digits and get 74. The first reel will stop on stop 74. We repeat the same process to get the stops for the remaining reels.

If this was a reel-spinning slot machine with only 22 physical stops per reel, there would be an additional step in the process. Let's redo the example for the first reel. The RNG tells us to stop on stop 74, but stop 74 doesn't exist. The output from the RNG tells us virtual stops, not physical stops. The program has to check a table in the program that says the physical stop that each virtual stop represents. This is the technique used to make the jackpot symbol appear much less frequently than the cherry symbol. There may be two cherry symbols and two jackpot symbols on a reel, but the jackpot symbol may appear only twice in the virtual reel table and the cherry symbol may appear 20 times.

Video poker machines use the output from the RNG differently from a slot machine. A video poker machine doesn't use the output from the RNG to generate an outcome, it uses the RNG to shuffle a deck. There are many shuffling algorithms. Here is an example of one. The program gets a number from the RNG and does some arithmetic on it to get a number from 1 to 52. Let's say the number is 21. The program then electronically takes the 21st card down from the top of the deck and puts it on the top of the deck. Then it repeats the process and moves the appropriate card to the top of the deck. The program continues doing this until someone presses the deal button.

When a player starts a game by pressing the deal button, the program takes the top five cards on the deck and displays them. The program then continues shuffling the cards remaining in the deck. When the player presses the deal button, the program takes the required number of cards off the top of the deck and fills in the discards with them.

Newer video poker machines take only five cards from the deck when you press draw. Older machines took 10 cards--the five needed for the deal and the five that might be needed for the draw. This old method was vulnerable to RNG cheats. A gang in the mid-West figured out the formula used by the RNG in a machine. They were then able to figure out where the RNG was in its sequence by entering the cards dealt and drawn into a computer program they had written. Once they had figured out where the RNG was in its cycle, they could use their program to determine what the draw cards were going to be. They were able to use this knowledge to win the maximum amount possible on each hand.

Key points about the RNG:

The program in a slot machine uses the output from the RNG to determine the symbols that will land on the payline on a slot machine.

The program in a video poker machine uses the output from the RNG to shuffle its electronic deck.

The RNG itself only generates numbers. It doesn't care what is done with those numbers. Thus, the same RNG can be used in a video poker machine and a slot machine.

Unless you have inside information about the formula used in the RNG, there is no way to determine what the next result on a machine will be given the current result.

Send your slot and video poker questions to John Robison, Slot Expert, at

For more information about slots and video poker, we recommend:

The Slot Expert's Guide to Playing Slots by John Robison
Break the One-Armed Bandits! by Frank Scoblete
Victory at Video Poker and Video Craps, Keno and Blackjack! by Frank Scoblete
Slot Conquest Audio Cassette Tape (60 minutes) with Frank Scoblete
Winning Strategies at Slots & Video Poker! Video tape hosted by Academy Award Winner James Coburn, Written by Frank Scoblete
The Slot Machine Answer Book by John Grochowski
The Video Poker Answer Book by John Grochowski
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