Is it possible to predict when a slot machine will pay out?

It is, or at least was, possible to predict when a slot machine will pay out. In a well-chronicled case, a criminal gang based in St. Petersburg, Russia successfully reverse-engineered the pseudo-random number generator (PRNG) employed by certain, older model slot machines, so that they could predict, with split-second accuracy, when a payout was due. The gang employed dozens of operatives, each of whom could reportedly profit by $250,000 a week, to exploit this vulnerability in slot machines in casinos in eastern and central Europe and in the United States.

The problem with a PRNG, as casinos discovered to their cost, is that results appear random, but are not, in fact, truly random. The algorithm, or set of rules, that generates pseudo-random numbers is initialised by a 32-bit integer value, known as a ‘seed value’; if the starting point in the sequence is known, the sequence can be reproduced at a later date.

The answer, as far as modern slot machines are concerned, was replacing the PRNG with a true random number generator (TRNG), which relies on atmospheric noise, rather than an algorithm, to generate random numbers. Consequently, it is impossible to predict when any modern slot machine, in a bricks-and-mortar casino or online, will pay out. The return to player (RTP) percentage, which describes what proportion of money wagered on a slot machine is returned to players over time, indicates what you can expect in the long-term, but not what to expect from one spin to the next.