Generator Of Random Numbers
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. It is a Random Number Generator that chooses the reels positions. It is included into the machine's software.
The RNG generates random numbers at an extremely high rate. 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 changes every fraction of a second.
The RNG was noticed to generate not exactly 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. A single period can't be completed by any computer in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. It was he who 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. 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. This is known as the theoretical payout percentage. The minimum theoretical payout percentage is controlled by jurisdictions. Law or regulation are used to establish it. There is a certain winning pattern 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 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. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. As it is a time-consuming process it is done infrequently. There is sometimes a tamper-evident seal on the EPROM. Only Gaming Control Board officials can change 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 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
A group of machines offers a particularly large prize, or jackpot if they are linked together in a special way. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. The amount of the progressive jackpot is far higher than any single slot machine's.
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 under computer software's control. It is possible to make the slot machine frequently display combinations that are close to winning combinations.
This practice is called near-miss programming.
A related phenomenon is sometimes called near-miss. 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. 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 issue of a near-miss above or below the pay line was also investigated by the Nevada Gaming Commission. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. In other words, any other combination must be just as likely to occur as the near-miss. Winning combinations aren't supposed to be shown 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.

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. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.