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A bookmaker offers sports betting with fixed odds on outcomes of future events. These odds can be changed by the bookmaker at any time. Completed bets, however, retain the agreed odds that was valid at the time the bet was made and can no longer be subsequently changed by the bookmaker.

The bookmaker always has the advantage, because the bookmaker never has to predict the right outcome. The bookmaker only needs to be able to correctly estimate the percentage of betting outputs. For this reason, the bookmaker needs extensive statistical materials and analysis.

The bookmaker also distinguishes between fair odds and real odds. If the odds are fair, the bookmaker would have to pay all the losers’ money out to the winners (when the events happen). From the real odds of the bookmakers, which of course are below, results in the profit of the bookmaker. The bookmakers of course do not say how much they pay, but you can determine the fairness of a bookmaker relatively easily. Roughly speaking, in a football game, the odds of winning, drawing and losing together are counted. The best and fairest bookmaker is the one with the highest sum.

However, to be exact, the odds are not counted directly, but the inverse of the odds (these are the implied probabilities), and the bookmaker whose sum of inverses is the lowest offers the best odds.