Generator Of Random Numbers
Nowadays slot machines are computerized. So 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. It is a Random Number Generator that chooses the reels positions. The machine's software includes a Random Number Generator.
Random numbers are constantly generated by the RNG at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. The result is different depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
The numbers generated by the RNG seem to be not exactly random. The sequence of numbers is repeated by most of the RNGs. Poor programming determines such behavior. It is relatively easy to build pseudo RNGs with very long periods. A single period can't be completed by any computer in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. Equations for specific gambling games like Keno were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the player cannot see the secquence.

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. The minimum theoretical payout percentage is controlled by jurisdictions. Law or regulation typically establish it. A certain winning pattern exists nearly in every casino. It contains the information about 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. 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. Changing the payout percentage requires a physical swap of the software. As it is a time-consuming process it is done infrequently. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. Gaming Control Board officials should be called for changing it. Other jurisdictions randomly audit slot machines to ensure 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 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.

Machines That Are Linked
A group of machines offers a particularly large prize, or jackpot if they are linked together in a special way. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. The amount of the progressive jackpot is far higher than any single slot machine's.
In some cases multiple machines are linked across multiple casinos. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss
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.
The term near-miss programming is used to denote this practice.
This term is also used for a related phenomenon. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. 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.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. That means that the possibility of near-miss occurrence should be equal to the possibility of any other combination. The machine can't show winning combinations more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. An example of modern ways of slot machine defrauding 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. The machine can begin paying in any second.
The player has either seen a person playing a slots game for a while with no payouts or alternatively they have been depositing funds into a slots game and haven’t yet seen any payouts. 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. Makes sense if you have lost 0. 00 and have yet to win. The machine will pay any way.
Many years ago players have their own way of determining whether the machine has not paid out for a long period of time. They touch the coins to see whether they were cold or hot. If the coins were hot they believed that the machine had not paid for a long period of time. But in reality any machine has numerous lights which heat the coins.
Chaser is the next type of slot players.
The player thinks that the machine he is going to play is not a machine that never pays. Players feel more comfortable and confident because they have been reinforced with a win and gain trust in the machine. One can feel the atmosphere of comfort and trust between the machine and the player.