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# Ask the Slot Expert: That can't be random

7 September 2022

Question: Can you better explain how a slot machine uses the random number generator to produce a spin result?

Let's take a 3 row x 5 column slot machine. I spin and get a full screen of the same symbol.

If a random number generator is used to select each of the 15 symbols shown, how could it possibly select the same symbol 15 times in a row? The odds of that happening even once in a lifetime seem staggering, but it happens again and again.

That can’t be random.

If I numbered 15 pieces of paper 1 to 15, put them in a bowl, drew one piece of paper out, checked the number written on it and it was 6, and put it back in the bowl, I can’t even fathom that I would draw that same piece of paper with #6 on it out 14 more times in a row.

Answer: Let's start with your scenario. You have a bowl of 15 slips of paper numbered 1 through 15. You draw a slip at random. It is 6. You put the slip back in the bowl. You then perform the same drawing 14 more times and each time you draw a 6.

How unlikely is that event?

You've drawn the first 6. The probability that you get a 6 on the next draw is 1/15. And on the next draw 1/15, and so on. The probability of drawing 14 6s in a row is (1/15) to the 14th power; 1/15=0.067, (1/15)^14=3.43e-17 or 0.0000000000000000343 or 1 out of 29,192,926,000,000,000 -- if I didn't make a mistake moving the decimal point. And even if I did, it doesn't matter. In your model, drawing 14 6s in a row is really, really (really, really, astronomically really) unlikely.

Yet, as you point out, it happens all the time.

We know that slot machines are heavily regulated and that they are tested to ensure that they operate properly and randomly. Either there is a country-wide -- global, actually -- conspiracy hiding the fact that slot machines aren't really random or ....

Let's look at another possibility. Our observations don't match what we expect to happen. Rather than doubting randomness, let's doubt our model.

This scenario is a bit closer to what's actually happening. We'll augment your scenario by adding a few slips to the bowl so we now have, say, 100,000 slips. They are still numbered 1 through 15, but we now have 40,000 slips numbered 6. The probability of drawing a 6 is 0.4. The probability of drawing a 6 14 times in a row is 0.00000268435456 or 1 out of 372,529. Kinda rare, but much more likely than hitting Megabucks or a mega-lottery.

The point of this example is that filling the screen is not a matter of choosing from 15 equally likely symbols. But we still don't have the right model.

A modern video slot operates much like a traditional reel-spinning machine on steroids. An old mechanical machine typically had reels with 24 stops. If it had only one jackpot symbol on each reel, the chances of hitting a jackpot are 1 out of 13,824 (24x24x24). That's okay if the top prize on a machine is, say, 1000 credits, but it just will not work if a machine wants to compete with lotteries and offer prizes in the millions of dollars.

Enter the computer. A stepper slot uses a random number generator (RNG) to choose where each reel will stop. Rather than choosing the stopping point directly, the computer program running the slot chooses from a virtual reel table.

In computer science, virtual means using something you have to make it seem like you have something that you don't have. You wear a virtual reality headset, for example, to make it seem like you're flying through the Grand Canyon or browsing the shop windows on Savile Row.

The virtual reel on a slot machines lets the program act as if the reels have 32, 64 or any number of stops the game designers wish.

To visualize a virtual reel, imagine that you took the reel strip on a machine and put Silly Putty on it to pick up the images. Then you stretched the Silly Putty so the bars and blanks take up more slots and you leave the jackpot symbols alone.

If our Silly Putty-enhanced reel is 32 stops long and we still have one jackpot symbol on each of our three reels, the chances of hitting the jackpot are 1 in 32,768 (32x32x32). (The slots that offer multi-million-dollar jackpots use virtual reels with many, many more stops.)

Even though a symbol may appear in multiple consecutive positions on the Silly Putty reel, all of the positions point to the same position on the physical reel.

I don't have a good segue here, so I'll just go for it -- Enter the video monitor.

When we get rid of the physical reel, the virtual reel becomes the reel. We can have reels as long as we want and we can distribute the symbols any way we want. We can place as many runs of three or more 6s on our reels as we want to make filling the screen with 6s much more likely than in your model.

To sum up, the machine has five reel layouts in its memory. The slot program uses a number from the RNG to pick where the first reel will stop. For example, if the number from the RNG was 37, the first reel would stop at position 37. The program then repeats the process for the other four reels. The program then evaluates the symbols displayed on the screen and, if you're lucky, locks up and calls a slot attendant to come with your handpay and tax form.

One other thing I should mention. Some machines have replaceable symbols, that is, some positions on the reels will all be replaced by a particular symbol chosen at random at the beginning of the spin. There was a machine at the Westgate that I was fond of playing that had this feature. I don't remember ever getting any big hits on it, though.

If you find you get full screens of different symbols frequently, the machine may have replaceable positions. You can check the help screens on the machine to find out for sure.

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