[edit] Gambling variables

There are three variables common to all forms of gambling:

How much is being wagered, the initial stake (in money or material goods).
The predictability of the event.
In mechanical or electronic gambling such as lotteries, slot machines and bingo, the results are random and unpredictable; no amount of skill or knowledge (assuming machinery is functioning as intended) can give an advantage in predictability to anyone.
However, for sports events such as horse racing and soccer matches there is some predictability to the outcome; thus a person with greater knowledge and/or skill will have an advantage over others.
The odds agreed between the two (or more) parties to the wager; where there is a house or a bookmaker, the odds are (quite legally) arranged in favor of the house.
The expected value, positive or negative, is a mathematical calculation using these three variables. The amount wagered determines the scale of an individual wager (bet); the odds and the amount wagered determine the payout if successful; the predictability determines the frequency of success. Finally the frequency of success times the payout minus the amount wagered equals the "expected value" The skill of a gambler lies in understanding and maneuvering the three variables so that the "actual value" is positive over a series of wagers.