Goal Expectancy Explained
Pillars Of Success: Understanding the significance of goal expectancy
In order to have any chance of being successful at any form of fantasy football you must be able to gauge the ‘softness’ of a fixture. If you are participating in seasonal contests, you will want to juggle your squad around so that players with the best fixtures on paper are in your starting XI on a weekly basis. It’s just as important for daily fantasy slates as well, since almost all fantasy football scoring systems award points for offensive and defensive actions, with goals scored and clean sheets highly significant.
There is a clear correlation between the number of points a player is expected to earn for any upcoming fixture relative to his team’s average tendencies. Each player will usually contribute a percentage of a team’s assists and goals so the more goals a team is predicted to score the higher a player’s points expectancy will be. It works in a similar way for defensive-minded players, so the less goals a team is expected to concede, the more points a goalkeeper or defender is likely to earn.
How do we calculate goal expectancy?
Well, you could build your own model but we don’t think that is necessary since a barometer already exists that can provide you with all the information you need – the betting odds provided by bookmakers. So how do odds compilers come up with their betting lines?
Bookmakers will use a model to predict the number of goals each side is expected to score in any given fixture. It will factor in the attack and defense strength of each team based on a sizeable sample of games that aims to give a clear reflection of each sides’ current ability as well as ground advantage, since we know that teams tend to score more goals in home games.
Adjustments will then be made for other factors that will include team news (injuries, suspensions and rotation risk), motivation (how important is the fixture), the playing conditions and so on.
Odds will then be released and the forces of supply and demand take over. Smart punters will bet if they see value and the bookmakers will react to these bets by adjusting their odds. By match-day all the information is readily available and the odds will near as possible reflect the true chances of each outcome.
Converting betting odds into expected goals
Armed with the latest match betting and over/under lines, we are able to carry out some straightforward calculations, effectively reversing the process, to come up with an expected goals figure for each team in any given fixture.
There is no need for you to build your own model or do the calculations yourself – we will present you with the data in our goals section, so that you have your very own fixture difficulty ranking.
Pigeonholing promoted teams can be tricky at the start of a new season but again, the weight of money usually talks, so don’t expect the bookmakers to be too far off. However, if you prefer a watching brief so that you can come up with your own conclusions then that is fine.
Sites such as WhoScored provide detailed analysis of a team’s characteristics so once you’ve factored in any pre-season transfer activity, you might be able to predict a rough idea of where you think they might sit in the table and use this as a base for assessing their future fixtures.
The power of Poisson
Poisson has been found to be a useful yardstick for calculating the probability of x number of goals being scored in a game, so now, since you have an expected goals figure for a particular team you are able to use the Poisson distribution to calculate the chances of them scoring 0, 1, 2, 3 or 4 goals and so on or even a combination of goals. Simply type a Poisson command into an excel sheet.
e.g Spurs have a goal expectancy of 1.75 and we want to know what the chance are of them scoring exactly 2 goals
=Poisson(2, 1.75, false)
Gives us a Poisson based probability of 26.6% that Spurs score exactly 2 goals, as a result of their 1.75 expectation.
We do appreciate that using excel or Poisson is not for everyone but once you start referencing team goal predictions, you will soon come to see how valuable they can be.
While the goalkeeper playing for the team with the lowest predicted concession rate has the best chance of a clean sheet, it’s not quite so simple for outfield players. You need to determine if a side is a ‘one-man’ band or whether they are more inclined to share the goals around. If it’s the former then a player representing a team with a high goal expectancy, is likely to be a prized fantasy asset for that particular gameweek. Some players will be directly involved in around 50% of all the goals their team scores – take note!
Successful line-up building starts with having the right foundations in place. An understanding of goal expectany is without doubt one of the key pillars to success. Make sure you incorporate it into your routine, so that you have a better chance of staying one step ahead of opposing fantasy football managers!