Random Number Generator
Nowadays slot machines are computerized. The odds are programmed. The reels and lever are present for historical and entertainment reasons only. It is a Random Number Generator that chooses the reels positions. It is included into the machine's software.
Random numbers are constantly generated by the RNG at an extremely high rate. When a player presses the Play button the most recent random number determines the result. That means that the result depends on exactly when the game is played. The result would be different a fraction of a second earlier or later.
It was noticed that the RNG does not actually generate random numbers. Most RNGs will repeat their number sequence. The cause of it is in poor programming. It is relatively easy to build pseudo RNGs with very long periods. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. Ronald Dale Harris, a former slot machine programmer, had access to the pseudo RNG code and seed values. He discovered equations for specific gambling games like Keno. Then the prediction was made that the next set of selected numbers would be based on the previous games played. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the player cannot tell where in the sequence they are.

Percentage Of The Payout
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. The term theoretical payout percentage is used to denote it. Jurisdictions influence the minimum theoretical payout percentage. Its establishment refers to the sphere of law or regulation. There is a certain winning pattern nearly in every casino. This is the amounts they pay and the frequencies of these pay-outs. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. As for the rest of the money it goes to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage requires a physical swap of the software. It is done infrequently because this is a time-consuming process. In certain jurisdictions the EPROM has a tamper-evident seal. Only Gaming Control Board officials can change it. The audit of slot machines is made by some jurisdictions make sure that they contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. After making the change the machine is locked to new players for four minutes, it displays an on-screen message informing potential players that a change is being made.

Linked Machines
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or jackpot. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. The amount paid for the progressive jackpot is usually far higher than any single slot machine could pay on its own.
Sometimes multiple machines form multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is controlled by computer software. That is why the slot machine can be programmed to display combinations that are close to winning combinations.
This practice of showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
A related phenomenon is sometimes called near-miss. It is winning percentages that control the chance of a winning combination appearing on a pay line. However, the combinations appearing above and below the pay line are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. This phenomenon only occurs where abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. It was considered to be legal as long as the near-miss above or below the pay line was not specially programmed. The near-miss and any other combination should have the same possibility of occurrence. Winning combinations aren't supposed to be shown more frequently than other combinations above or below the pay line.
Near-miss programming, where a near miss is inaccurately displayed is not allowed in Australia either. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Slots Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors were changed for bill acceptors. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. Nowadays microwaves are used to defraud slot machines.