Regardless of your ethical standing on gambling, there is rich and interesting information to be had in football betting as evidenced by the extensive research in fields ranging from pure statistics to machine learning to come up with approaches to best the bookmaker.
Odds represented in decimal form are most prevalent in Europe. Decimal odds can be considered as multipliers for your money. If you bet $1 on odds of 2.5 and win, you take back $2.5. You can convert decimal odds to fractional odds by subtracting 1 from the decimal odds and converting the result into its simplest fraction.
Eg. To get the fractional odds for 2.5:
2.5 – 1 = 1.5. The simplest fraction for 1.5 is 3/2, which is the required fractional odds.
You can get probability from odds by taking the reciprocal of the odds (represented in decimal form). Theoretically, if the odds are 2.0, it means that the probability of the event occuring is 1/2 = 50%.
In slot machines, ‘theoretical payback’ refers to the percentage of the total wagered money that should be returned to the players. When used for a good amount of time, the actual payback of slot machines almost mirrors the theoretical payback promised by the machine designers. Theoretical payback exists in sports betting as well, and can be detected in a simple way.
Calculate the probabilities of all the outcomes in the market (probabilities of win, draw and loss in the 1×2 market, for example) and add them up. This should yield 1, but you will end up with a figure higher than that. This is due to the effect of theoretical payback. You can calculate the theoretical payback by multiplying the odds offered with the probability.
You can now obtain the true bookmaker probabilities considering theoretical payback: Probability = theoretical payback/odds.
A more formal presentation of this introduction can be found in this master’s thesis by Rasmus Olsen.