Simply put, goal expectancy is the number of goals we expect a team to score in a given match given their own potential to score, potential to concede and the potential of their opponents to do likewise. Of course, how we calculate this is a matter of serious debate and largely the topic of this article.
While the subject of goal expectancy has been discussed enthusiastically in recent years, with everyone from casual football forum posters to highbrow academics each having their say on the topic, all that we must understand here is that the more accurately we predict the goal expectancy of each club in a given football match, the more likely we are to find value bets and as a result, earn consistent profits from your football betting. Read more betting tips in footyguru365.
In short, the profitability of any football betting model hinges on its capacity to forecast accurate scorelines match by match and in turn, translate those forecasts into betting odds. This is where what is known as Poisson Distribution comes into play.
What is Poisson Distribution?
In essence, Poisson Distribution calculates the probability of each possible scoreline in an upcoming football match given the goal expectancy for each club competing in the match.
The intricacies of the Poisson Distribution need not be fully understood for you to make use of it, because Microsoft’s Excel has a built-in Poisson function. In statistical terms, the formula for Poisson in Excel is:
=POISSON(x, mean, cumulative)
Mean is our goal expectancy for an individual team. We must also set ‘cumulative’ to FALSE, which results in POISSON returning the probability that a random variable, in this case goals, takes on a value exactly equal to x.
In other words, if you want the probability that a team will score 2 goals in a given match, and your calculated goal expectancy for that team in this match is 2.127 goals, then your formula is:
The output from this is .2696 – i.e. there is a 26.96% probability of this team scoring exactly 2 goals in this match.
To derive meaningful football match odds, we need to know the probability for all goals though, or at least all likely goals. In the example below we have calculated the likelihood for both Arsenal and Sunderland scoring exact goal totals given their pre-match goal expectancy. For this match the pre-match goal expectancy (based on a hypothetical model) were:
- Arsenal goal expectancy = 2.12673
- Sunderland goal expectancy = 0.75001