StrataBet currently provides model prices for matches in 17 competitions around the world, but does not account for team news or other factors that influence a football match (Models FAQ – “What ‘data’ is not included in the model?”).
Using my own experience as a football trader, I will attempt to show how to use the StrataBet player ratings to account for team news on a very basic level.
The first part of the process is identifying the strongest XI that each team could field if everyone was available for selection. This can be quickly calculated by viewing player ratings from the most recent Preview, in the “Expected Line-Ups” tab.
If we look at Ross County in the Scottish Premiership, we can list their strongest XI as follows:
It would not be correct to assume that if Gardyne RW3 and Robertson RB/CB3 were both absent for the next match, you would deduct a value from Ross County’s Expected Goals. Instead, if we think of each team as two units (an attacking one and a defensive one) we can accurately account for the weakening effect of important players being absent (Supremacy), and also the impact on the expected goals in the game (Total Goals).
In general terms, if an important striker is missing for a game, you would expect his team to see a lower goal production than average. Naturally, the opposite is true for a missing goalkeeper or key defender.
Goalkeepers (GK), full-backs (RB/LB), centre-backs (CB) and defensive midfielders (DM) belong to the Defensive Unit. Strikers (ST), support-strikers (SS), attacking midfielders (AM) and wingers (RW/LW) belong to the Attacking Unit. Central midfielders (CM) and wide midfielders (RM/LM) are in neither group, as they are considered to contribute equally in both defence and attack.
If we apply this rationale to Ross County we get the following:
To give a real-world example here: you have model prices for a match, but on reading the Preview you notice that each team is missing some players from their Strongest XI. What do you do?
From the Preview, Ross County are replacing the injured Gardyne RW3 with Franks RW1 and Boyd CB1 is expected to start over Robertson RB/CB3.
On a very basic level, a quick calculation you could make would be to take the difference in the ratings, multiply by 2 and divide by 100. This would give:
RW1-RW3 = -2 * 2 / 100 = -0.04
CB1-CB3 = -2 * 2 / 100 = -0.04
On the face of it Ross County are weakened by 0.08 of a goal for this match.
You would then go through the same process for their opponents and add the net figure onto the Supremacy quote. For example if Ross County were away from home, their opponents were at full strength and the model Supremacy quote was 0, then the new quote is 0.08.
Total Goal Adjustments
Using the same team news as above, we can see that one of the missing players belongs to the Attacking Unit and the other to the Defensive Unit.
We would use the same adjustments as the above example to arrive at 0.04 for each player change, but this time the Defensive change is a +0.04 adjustment onto the Total Goals, and the Attacking change is a -0.04 adjustment onto the Total Goals.
In this case, they cancel each other out. Again, you would complete this task for Ross County’s opponents to arrive at a net figure and then add/subtract onto the Total Goals quote.
Other things to consider
As I have mentioned, this is a very basic way of looking at adjusting model prices for team news.
It does not take into account the match prices. If Barcelona were -2.5 favourites at home to Real Sociedad, then they are expected to attack for most of the match. This would have an impact on how much we would adjust if Messi ST5 was missing, or if Busquets DM4 was out.
It is obviously correct to adjust for important players being suspended or injured, but what if the manager simply decides to start a lower rated player over a higher rated one? If we trust his judgement, considering he sees every player in training each day, then it may not merit a negative adjustment at all.
These are some of the issues that will be explored in further detail in future blog posts.