Generator Of Random Numbers
Modern slot machines are computerized. They programme the odds. The reels and lever are present for historical and entertainment reasons only. The positions of the reels are chosen by a Random Number Generator. The machine's software contains it.
The rate of generating random numbers by the RNG is extremely high. When a player presses the Play button the most recent random number determines the result. This means that the result varies depending on exactly when the game is played. The result is different every fraction of a second.
It was noticed that the RNG does not actually generate random numbers. The sequence of numbers is repeated by most of the RNGs. The cause of it is in poor programming. 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. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. 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.

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 payout percentage is called theoretical. The minimum theoretical payout percentage varies among jurisdictions. It is typically established by law or regulation. A certain winning pattern exists nearly in every casino. 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.
A slot machine's theoretical payout percentage is set at the factory when the software is written. 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. One can find a tamper-evident seal on the EPROM in certain jurisdictions. Gaming Control Board officials should be called for changing it. 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. Only after the selected machine has been idle for at least four minutes can the change be done. 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.

Linked Machines
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked 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. A single slot machine's jackpot is considerably less than the progressive jackpot.
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
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.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
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. 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. In video slot machines 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. They ruled that this was legal, so 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. The machine can't 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.

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 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. Nowadays microwaves are used to defraud slot machines.

Here is the list of the most amusing myths concerning slots.
You can spot a winning slot machine.
The only way you'll spot a winning slot machine is when the bells are ringing and the money is dropping. Slot machines use random numbergenerator and that's exactly that. Random.
Casinos place loose slots near the cashier, aisle ways, etc.
If this were true, it would be the only machines anyone would play. A casino mixes slots all over the casino floor. No one with any experience at playing slots would make a statement like this.
Count the number of symbols on the reels to determine your odds.
This is mathematically impossible, remember the RNG (random number generator) determines the stops. Thereare literally millions of combinations on a 3 reel slot machine, so there is no way of determining odds by the symbols on the reels alone.
Casinos can tighten or loosen slot machines from the back office.
It is not as simple as that. The casino can’t just change the chip of the slot machine. It is illegal and it has to be permitted by the commission.
A slot machine that has not been paying is due to hit.
Absolutely false. The RNG assures this. You have an equal chance of winning on a losing machine as you do on a machine that just paid out the jackpot.
Pulling the slot machine handle produces more wins than pressing the spin button and visa versa.
It is also a mistake. These things are only mechanisms for beginning the game. The machine knows whether you win or lose before the symbols begin to stop.
Playing with hot coins improves your chances.
Once again, the computer determines a win or loss and the temperature of the machine or moneyirrelevant. Although it's funny that people still burn their fingers with lighters believing this works.
Casinos set slot machines to payback more on the weekends.
Wrong again. Your odds of winning are the same no matter if it's 4:00pm Monday, 8:00pm Saturday or 3:00am Wednesday. Casinos can not and do not do this.

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.
When the first slots only appeared cheating was impossible because they worked mechanically, with independent wheels spinning and then stopping randomly. Modern slots are much different. There is a chip inside of every slot machine, this chip is programmed to select numbers randomly. After selecting a number a corresponding symbol appear on the screen. There is a computer code. It generates the random numbers and determines whether the slot machine is loose or tight. The gaming inspector will examine the machine to make sure it will not deceive the player. That is, inspectors make sure that the machine pays off at the legal rate or better.
Because slot machines don’t pay off with every spin, it’s difficult for customers to tell whether a slot machine’s chip has been programmed to pay off less often than is legally required. The machines have to be examined before people begin to play them. Of course it is very difficult to examine every single machine, that’s why inspectors examine only brands. In the case of the approval of the brand the manufactures can produce the slot machines, even change them if it doesn’t go out legal bounds. Manufacturers can change chips to make a machine pay less as long as the inspectors have approved that particular chip.
In one of the casinos the chip was programmed in such a way that royal flushes would never come out. Or a chip that shows near misses of a jackpot. The effect was two-fold: the machine didn’t pay off, and customers continued to play the machine, thinking that they were very close to a big jackpot. If you think that you have come across such a machine you have a right to complain. Inspectors will respond by pulling the slot machine’s chip and seeing if it is an approved chip. Manufacturers that cheat are fined or they can lose their license.