Data is becoming more and more available in football and is now largely accessible to the average fan. However, as is clear to most people, not all data is useful.
Shot data is a prime example of this.
The standard way of recording shots on target or off target can often give a misleading view of what happened during a game, particularly if you haven’t seen the shots in question.
In StrataBet, we don’t just record shots on/off target. We record the quality of chances.
Rich Huggan did an excellent job of explaining that here, but to summarise:
- A “Great Chance” is a situation that a player would be expected to score from.
- A “Good Chance” is a situation that a player could score from but would not necessarily be expected to.
- An “Attempt” is a situation that a player would not be expected to score from.
This is simple, but extremely effective.
Rich’s blog gave detail on the conversion rates of these chances. It showed a linear trend that provides an excellent basis to investigate which teams are over-performing or under-performing.
This season I have been working on the Scottish Premiership, so have taken the chance to look at which teams have been “good” and which have been “lucky”.
The following matrix compares the “Difference in Expected Goal Difference”, using StrataBet chance data to determine whether a team has more or less goals than expected, and “PDO”, a metric in Football Analytics that is essentially 10 x (Shooting Percentage + Save percentage) – further details available on James Grayson’s blog here.
The matrix is split into 4 quadrants – to the right of the vertical axis are teams who have outperformed their Expected Goal Difference. These teams have made more of their chances than would be expected, while teams to the left of the axis have not scored at a rate consistent with the chances they have produced.
Being above the horizontal axis means a team is performing well – scoring lots of goals and/or saving a high number of shots on target. This is likely to regress as the season continues, as it is believed to be unsustainable.
Teams in the top right quadrant have a better goal difference than the expected chances they have created/conceded, while they also have a higher than expected Shots on Target and Saves ratio. Teams in the lower left are the opposite.
Those in the upper left quadrant have a higher than expected shots/save ratio but have not scored as many/have conceded more than would be expected from the chances against. The opposite is true of the bottom right.
To take a team as an example, Dundee United are massive outliers in the bottom left corner and currently sit in 12th place. They have a Difference in Expected Goal Difference of -14.
When comparing them to St. Johnstone (4th place, Difference in Expected GD of +9) we see that Dundee United have created just 4 less Great Chances than St. Johnstone and have had more Good Chances (+10)/Attempts (+12), giving them an identical Expected Goals For (18). However, St. Johnstone have actually scored 29 while Dundee United have only scored 10.
They have faced exactly the same number of Great Chances while Dundee United have conceded more Good Chances (+14) but less Attempts (-10). Dundee United would have been expected to concede 22 goals while St. Johnstone would have expected to concede 20. In actuality Dundee United have conceded 28 and St. Johnstone 22.
Of course this is just one example, but in general this kind of information is key in providing insights into which teams are underperforming and likely to improve, while also highlighting which teams are likely to decline as the season progresses.