Football analytics is a young business. And as such, it is still a rapidly developing field, where new concepts are launched all around. Some of these concepts are there to stay, others disappear as quickly as they came. For me, Game States definitely belong to that first group.
With Game State we indicate the score differential of the match in-play. Each match opens with both teams at GS 0, and a scoring team moves to GS +1, with the conceding team moving to GS -1. This Game State obviously has a big influence on how teams approach the game at hand. However, in traditional – if I may say so in this young business – football analytics groups all match events together, regardless of Game State.
The best concepts in football analytics make rational sense as well as intuitive sense. And such is the case with Game States. A team holding a narrow lead is a different team than a team that defends that lead. Obviously, better teams hold more leads than they defend, but even within teams, the shifts that occur when Game States change are fairly homogeneous. We’ve learned before that moving from GS 0 to GS +1 brings an average team a 10% decrease in Total Shot Rate, while the opponent increases 10% simply because of the shift in Game State.
On this day before the big game, PSV – Ajax, we look at the two best teams of the Eredivisie with a focus on their performance levels at the most crucial Game State: GS 0. The main reason for doing so, and I can safely say this out loud now, is that I have my doubt about the accuracy of the Total Shot Rate model used to predicted the final Eredivisie standing. It has significantly overestimated PSV and underestimated Ajax.
The model uses the Relative Shot Rate (RSR) to estimate the total points at the end of the season. RSR is a variation on the Total Shot Rate (TSR). Early in the season, the RSR has advantages over TSR, because teams have encountered a different strength of opposition, but by now those advantages have gone and RSR is nearly equal to TSR. At the moment, PSV’s TSR stands at 0.671 with Ajax at 0.632. Now what does this figure tell?
PSV has a higher ratio of chances created and conceded. Does this single figure make PSV the better team? No, because you may generate all the chances you want, you’ll need conversion as a skill to turn shots into goals.
PSV’s shooting percentage stands at 17.0%, which compares favorably to Ajax’ 15.4%. Does a higher TSR in combination with a higher shooting percentage make PSV the better team? No, because you can score all you want, you’ll need to prevent the opponent from scoring from their shots too, and this is where saving percentage comes into play.
PSV’s saving percentage is 87.4%, compared to Ajax’ 89.5%. But wait, that’s about the same difference as we found at shooting percentage, only this time PSV comes out on top. That’s true, and so both teams have a comparable PDO, which is the sum of shooting percentage and saves percentage. PSV’s PDO is 1044, and Ajax’ PDO is 1048.
|Performance metrics at all Game States|
So, if we wrap these numbers up we can safely say that PSV generates a higher ratio of shots. Taking shooting and saving into account, both teams are roughly equally efficient. Now why doesn’t PSV live up to the expectations of our TSR model?
The answer is to be found in game states. We can repeat the exact same exercise of looking at shot rate, shooting percentage and saves percentage for each game state. I won’t go over every single number, but instead focus on the most crucial Game State: GS 0. The average Eredivisie team plays out nearly 50% of shots at this Game State, but since Ajax and PSV are the two top teams, they can be expected to play out less shots at GS 0. Of all shots in Ajax’ matches, 41.9% take place at GS 0. For PSV this number is 35.4%.
Here’s the table for PSV and Ajax in terms of TSR, shooting and saving efficiency, and PDO at GS 0. Note that PDO in this case provides a nice summary of efficiency, wrapping up both offensive (shooting) and defensive (saving) skills.
|Performance metrics at GS 0|
The TSR tells us that at the most crucial Game State (GS 0), Ajax is by a distance the better team in terms of shot creation. PSV partially makes up for the lower TSR with their shooting percentage of 15.1%, which is higher than Ajax’ 12.8%. However, PSV loses this advantage in saves percentage, because their 86.7% is much lower than Ajax’ 92.4%. The combined efficiency is higher at Ajax, indicated by a PDO at GS 0 of 1052, compared to 1018 for PSV.
So, analyzing all shots in every match in one group, PSV seems the better team.
But at GS 0, the most crucial stage of the match, Ajax creates a better shot ratio, and is more effective. They gain more leads, which is a good thing in itself, but it also allows them to play more time at favorable game states, leading to an even better performance.
This post is a translation of yesterday’s article for ‘De Zestien’, the football blog of Dutch national newspaper ‘De Volkskrant’. Admittedly, it turned into a rewrite, more than a translation.