The RNG
There are many modern computerized slot machines. 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. The positions of the reels depend on a Random Number Generator. 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. 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.
Some professional gamblers observe that the RNG does not actually generate random numbers. It turned out that The number sequence is repeated time after time. This behavior is due to poor programming. Pseudo RNGs with very long periods canbe built relatively easily. There is no computer that is able to complete a single period 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. The fact that the next set of selected numbers would be based on the previous games played was revealed. 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.

Payout Percentage
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. This payout percentage is called theoretical. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation typically establish it. A certain winning pattern exists nearly in every casino. It concerns 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 players get the rest of the money.
At the factory when the software is written a slot machine's theoretical payout percentage is set. Physical swap of the software is needed for changing the payout percentage after a slot machine has been placed on the gaming floor. Being a time-consuming process it is done not very often. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. Gaming Control Board officials should be called for changing it. According to other jurisdictions slot machines are randomly audited 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 the change is made, the machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made.

Machines That Are Linked
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. 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. A single slot machine's jackpot is considerably less than the progressive jackpot.
Sometimes multiple machines form multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
Computer software controls the reel display of modern slot machines. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
The term near-miss programming is used to denote this practice.
This term is also used for a related phenomenon. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. The combinations 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 pay line than on the pay line. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. The symbols that appear around the winning line in video slots are 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. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. 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 near-miss programming, where a near miss is inaccurately displayed is not allowed either. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Slots 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. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.

You must know exactly the game you are playing.
Only the correct amount of money should be inserted into a machine. Before leaving a machine cash out your credits.
Slots shouldn't be a stress for you, just enjoy. You can increase your bets only when you are winning, and decrease your bets when you are losing.

You can be sure that slot machines are designed to be honest. But there are some manufactures that design the slots in such a way that they can deceive players.
In the old days, slot machines worked mechanically, with independent wheels spinning until they each stopped randomly. Modern slots are much different. Now the integral part of any slot machine is a chip, which is programmed to select numbers randomly. After selecting a number a corresponding symbol appear on the screen. The computer code that generates the random numbers is responsible for whether the machine is “loose” or “tight,” so gaming inspectors look at the machine’s code to make sure that a slot machine doesn’t cheat. The machine has to pay out either to the legal rate or better. Of course the latter happens very seldom.
It is next to impossible to say whether the chip is programmed not in a right way because the machine is not supposed to pay out with every spin. Before the machines are installed in casinos they are examined by inspectors. But you must understand that inspectors can’t examine every machine. They examine the brands. Once inspectors approve a brand of machine, the manufacturers can make changes to individual machines as long as the changes fall within the legal bounds. Some dishonest manufactures use the right to produce machines, but use a different chip, which is programmed to pay less or not so often.
For example, the Nevada Gaming Commission caught American Coin using chips that did not allow poker royal flushes. Similarly, Universal Distributing Company was caught programming machines to show a “near miss” of a jackpot. One the one hand the machine would never have a jackpot and the player would keep on playing. If you think you’re playing a machine that is not honest, you can complain. Be sure that inspectors will examine this very machine and the casino itself to make sure everything is ok. Manufacturers that cheat are fined or they can lose their license.