An Internet bookmaker has more flexibility in this respect, and can change a price to manage his projected liability. Nevertheless, with the exception of the most popular football leagues like the English Premiership, betting odds for the majority of matches remain unchanged up until kick-off. Even for high profile games like Manchester United versus Arsenal, where bookmakers can expect to see a large turnover, the odds available for the standard home/draw/away market may not fluctuate by more than about 10% from the original prices. Prices for other fixed odds football betting, including correct score, double result and total goals rarely change at all.
Fair odds are simply defined by the ratio of the amounts two parties would choose to risk, or stake, on the outcome of sporting a sporting event. This ratio is determined by the estimated probabilities of each outcome occurring. In an England v Scotland football game, for example, if it is estimated that England have an 80% chance of victory, the size of the stake for an England win would be 4 times that for the stake for England not to win (or 80%/20%). If England win, the bettor backing England would win £20 for an £80 stake. In England fail to win, the bettor backing England not to win would return a profit of £80 from a £20 stake. Consequently, the odds for an England victory would be 1/4, and 4/1 fo England not to win. The expected probability of all possible betting outcomes will total 100%. "Odds" is really just a betting term for probability. Of course, in football match betting, odds are also offered for the draw.
A bookmaker is a person or business that provide an odds market for one or more events, with prices available for all possible event outcomes, adjusted according to the demand of the bookmaker's customers, the punters. "Bookmaking" technically refers to the management of betting probabilities for the purposes of making a profit over a large number of events for which odds are offered. A "book" is simply the full record of betting transactions at all the available odds made with the punters for a particular event.
Since the bookmaker's aim is to make a profit, he will never offer a book where the expected probability of all possible betting outcomes on a single event totals 100%. By reducing the odds for each betting outcome, the totality of expected probabilities is increased above 100%, thereby ensuring that, if managed correctly, the book will make a profit for the bookmaker. The size of this expected profit margin is sometimes referred to as the bookmaker's overround. Although in this sense bookmakers' odds are unfair odds, overround betting was actually introduced in the early 19th century to remove the necessity to cheat. The standardisation of the bookmaker's profit margin according to mathematical principles effectively professionalised the betting industry.
Odds of 4/1 are sometimes called fractional odds. More frequently on the Internet today bookmakers make use of decimal notation to express the odds of various available wagers. It is fairly straightforward to convert from fractional odds into decimal odds, because the size of the fractional odds represents the potential profit from a winning bet.
Decimal odds = Fractional odds + 1
For fractional odds of 9/4, the value of the decimal odds will be given by:
(9/4) + 1 = (9/4) + (4/4) = (13/4) = 3.25
Usually, decimal odds are rounded to the nearest 2 or at most 3 decimal places. Consequently, fractional odds of 10/11 would be rounded to 1.91 (since 21/11 = 1.909090909…).