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Best of Donald Catlin

Gaming Guru

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Picking the Right Slot

2 September 2007

You haven't seen much about slot machines in this column for the simple reason that I seldom play them. Why? Slot machines are not very good bets. Typical payback percentages are 96% (rare) down to 83% (typical of airports). 92% - 93% is common. In spite of this, slots are popular, enjoying the major portion of casino floor space. So, since many of you out there are slot players, let me give you my take on picking the right slot.

My premise is that you would like to find the machine with the highest payback percentage. What is more, you would like to walk out of the casino with most of your money intact or even with a small profit. This is a tall order. To this end let's begin with a few facts.

In the early days the reels on a slot machine stopped randomly and the frequency of paying configurations was determined by the number of each type of symbol on each reel. So one could, through observation, determine the reel configuration, though not without considerable effort. This combined with the payout schedule could enable one to determine the payback percentage. Since 1983, however, this is no longer possible. That was when the notion of a virtual reel came into being. Here's how it works.

Every slot machine comes with a random number generator (RNG) and an EPROM (erasable programmable read only memory) that contains, among other things, a table of numbers called a virtual reel. Each number corresponds to one of the stopping configurations of the reels. The RNG is constantly spitting out one of these numbers in a random sequence. When you drop your coin, press the play button, or pull the handle (depends upon the machine) you get whatever number is coming out of the RNG at that instant. That number is sent to the EPROM which in turn sends a message to the reels telling them what the stopping position is that corresponds to your number. They spin and then stop in that configuration. Two identical machines with identical pay tables can contain two different EPROMs, one with a virtual reel that returns 98% to the player and the other returning 83%. The symbol configuration on the reels provides no information about the payback percentage.

That is the bad news. Here, however, is a bit of good news. Occasionally a casino will provide you with information about a machine's payback percentage. One must be careful though. If a sign on a machine says "This machine returned over 100% last week," that is completely useless information. What happened last week has no bearing on the present. If a machine has a sign that says "This machine returns 97% to the player," that is useful information, at least in Nevada. (I'm not going to consider other states or Indian casinos - I just don't know what the story is in these venues.) In Nevada if a casino states a payback percentage then by law it must be the truth. Unfortunately, you probably are not going to see a payback percentage stated for a particular machine. What is quite common, however, is to see a payback percentage listed for a carousel of machines. Here again one must be careful. If the sign says "These machines return up to 98% to the player." but that "up to" means that at least one of the machines returns 98%; perhaps only one. What you want to look for is a sign saying "These machines return 98% to the player." By Nevada law every one of the machines in that carousel must, on average, return 98% to the player.

If you're lucky enough to find such a carousel, then the next step is to check each of the machines and select the one with the lowest jackpot payout. Why lowest? The reason is that if a jackpot payout is unusually large (1) it is hard to hit and (2) it means the rest of the winning combinations have to have lower payouts to support this large jackpot. If you play a machine with a smaller jackpot (which you probably won't hit) your chances of getting a decent payback on the smaller payouts is increased.

Is my advice a formula for success? No indeed! Playing slot machines makes it very likely that in the long run you will lose money. But my advice will generally give you longer playing sessions and perhaps even a few sessions where you walk away with a small profit. So good luck and I'll see you next month.


Don Catlin can be reached at 711cat@comcast.net

Donald Catlin

Don Catlin is a retired professor of mathematics and statistics from the University of Massachusetts. His original research area was in Stochastic Estimation applied to submarine navigation problems but has spent the last several years doing gaming analysis for gaming developers and writing about gaming. He is the author of The Lottery Book, The Truth Behind the Numbers published by Bonus books.

Books by Donald Catlin:

Lottery Book: The Truth Behind the Numbers
Donald Catlin
Don Catlin is a retired professor of mathematics and statistics from the University of Massachusetts. His original research area was in Stochastic Estimation applied to submarine navigation problems but has spent the last several years doing gaming analysis for gaming developers and writing about gaming. He is the author of The Lottery Book, The Truth Behind the Numbers published by Bonus books.

Books by Donald Catlin:

Lottery Book: The Truth Behind the Numbers