Random Number Generator
Nowadays slot machines are computerized. They programme the odds. In modern slot machines, 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.
The rate of generating random numbers by the RNG is extremely high. When you pres the Play button the result is determined by the most recent random number. 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. The sequence of numbers is repeated by most of the RNGs. Poor programming determines such behavior. To build pseudo RNGs with very long periods is relatively easy. There is no computer that is able to complete a single period in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. It was he who discovered equations for specific gambling games like Keno. They allowed to predict that the next set of selected numbers would be based on the previous games played. However even if nobody uses the machine the RNG continues to pick numbers. So the player cannot tell where in the sequence they are.

Payout Percentage
Slot machines usually 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. It is typically established by law or regulation. A certain winning pattern is determined by every casino individually. The amounts they pay and the frequencies of these pay-outs are reflected in it. The selection of the slot machines winning patterns is chosen in such a way that it could yield a certain fraction of the money played to the house. The rest of the money is given back to the players.
When a slot machine's software is written its theoretical payout percentage is also set. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. It is done infrequently because this is a time-consuming process. One can find a tamper-evident seal on the EPROM in certain jurisdictions. It can only be changed in the presence of Gaming Control Board officials. Other jurisdictions constantly ensure that slot machines contain only approved software.
The technology being developed by the Nevada Gaming Commission would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change can be done 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
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. A single slot machine's jackpot is considerably less than the progressive jackpot.
There can also be multiple casinos consisting of multiple machines. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss Programming
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.
Such 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. Winning percentages programmed into the slot machine 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. This means it is much more likely that a winning combination will appear above or below a pay line than on the pay line. Only if abbreviated physical reels are used to display a win pattern based upon the RNG this can occur. Video slot machines have virtual reels and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The Nevada Gaming Commission investigated 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. In other words, the near-miss must be just as likely to occur as any other combination. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. To audit manufacturer's practices regulators use stop motion cameras.

Slot Machines Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.

There are at least two types of slot players.
The first type is a persistent player.
The players of this type believe that very soon there will be a period when the machine begins to pay. Just a second and the coins will be flying all over the casino!
Players become so persistent if they have already spent a lot of money for this slot and haven’t got anything or if they have found a slot machine that haven’t paid for a long period of time. They believe that a big win is sure to happen right now. They can’t leave the machine because as soon as they do it will burst forth and pay out large sums. Maybe it can really work if you have already spent thousands of dollars and have got nothing back. The machine will pay any way.
Many years ago in the case of the payout players touch the coins to check whether he coins were hot or cold. If the coins were hot they believed that the machine had not paid for a long period of time. The heat of the machines lights, etc would eventually heat the coins.
The second type of slot players is a chaser.
The player believes the slot machine is not one of those machines that never pay. Players feel more comfortable and confident because they have been reinforced with a win and gain trust in the machine. A sense of comfort exists between the slot machine and the player.