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Random number generator
It is a common belief that the odds on a machine have something to do with the number of each kind of symbol on each reel, but in modern slot machines this is no longer the case. Modern slot machines are computerized, so that the odds are whatever they are programmed to be. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions the reels will come to rest on are chosen by a Random Number Generator (RNG) contained in the machine's software.
The RNG is constantly generating random numbers, at a rate of hundreds or maybe thousands per second. As soon as the lever is pulled or the "Play" button is pressed, the most recent random number is used to determine the result. This means that the result varies depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
Payout percentage
Slot machines are typically programmed to pay out as winnings between 82 to 98 percent of the money that is wagered by players. This is known as the "theoretical payout percentage". Suppose that a certain slot machine costs $1 per spin. It can be calculated that over a sufficiently long period, such as 1,000,000 spins, that the machine will return an average of $950,000 to its players, who have inserted $1,000,000 during that time. In this (simplified) example, the slot machine is said to pay out 95%.
A slot machine's theoretical payout percentage is set at the factory when the software is written.
In many markets where central monitoring and control systems are used to link machines for auditing and security purposes, usually in wide area networks of multiple venues and thousands of machines, player return must usually be changed from a central computer rather than at each individual machine. A range of percentages are preprogrammed into the game software and selected by configuring the machine remotely.
Linked machines
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or "jackpot".
In some cases multiple machines are linked across multiple casinos. In these cases, the machines may be owned by the machine maker who is responsible for paying the jackpot. The casinos lease the machines rather than owning them outright.
Slot machines that are not linked to a large regional jackpot usually have higher payout percentages, as linked machines have to take into consideration the large jackpot amount into their payout percentage calculations.
Near-miss programming
Because the reel display of modern slot machines is controlled by computer software, it is possible to make the slot machine frequently display combinations that are close to winning combinations. For instance, if the jackpot combination is "7-7-7", a slot machine could be programmed to frequently display "7-7-(non-7)".
This practice of showing combinations that are similar to winning combinations more frequently than would occur randomly is called "near-miss" programming.
There is a related phenomenon that is also sometimes called "near-miss". The chance of a winning combination appearing on a payline is controlled by the winning percentages programmed into the slot machine. However, the combinations appearing above and below the payline are all roughly equally randomly distributed. This means it is much more likely that a "winning combination" will appear above or below a payline than on the payline. Using the same example above, it is much more likely that a "winning combination" of "7-7-7" would appear on a line above or below the payline than the chance that it would appear on the payline. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. In Video Slot Machines, virtual reels are used and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
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