Sometimes the easy questions can be the hardest ones to answer correctly. This is true in statistics, and since we apply numbers to football, this is true in football analytics as well. Take the never ending debate around shot conversion. Why are better teams able to convert a higher percentage of their shots into goals?
Providing an answer is not the hard part here. Providing a correct answer is.
Let try this, often heard answer. It’s simple. Better teams have the better players. Better players hit more difficult shots, leading to more goals per shot.
The problem with this answer is not that it isn’t correct. Because it is. Better teams have better players, and these players turn more shots into goals than weaker players.
The problem with this answer is that it stops most people from looking beyond it and consider other factors that come into play here. And you’d probably guessed from the title of this article already, that it’s game states and conversion that I would like to link today. It turns out that game states may well explain more of the variation in shot conversion between better and weaker teams than player quality will ever do.
Two weeks ago I wrote about Total Shot Rate (TSR) and Game States. Let’s recall the graph that was central in that piece.
Let repeat this exercise for shot conversion. So, here’s the same graph, linking shot conversion and game state. Please note that this graph contains all shots from all Eredivisie teams in the present season until match day 27. This time I’ve concentrated on GS -2 to GS +2, to prevent the low numbers at more extreme Game States from disturbing the picture. The shot numbers at different Game States are 371 shots at GS -2, 1160 shots at GS -1, 2885 shots at GS 0, 922 shots at GS +1 and 363 shots at GS +2.
It turns out that, like TSR, shot conversion is also related to Game State. TSR had a complicated shape, with an inverse correlation at close GS, but shot conversion is a lot easier to digest.
In general, the more favorable the Game State, the better shot conversion is. The only exception is at GS 0, where shot conversion is lower than at GS -1 and GS +1. Overall shot conversion for the league is 11.7%. Shot conversion at favorable Game States is significantly better, with GS +1 at 14.1% and GS +2 at 17.1%. The most interesting observation is that shot conversion at GS 0 (10.4%) is lower than at GS -1 (11.6%).
Things become more interesting when we combine the conclusions from TSR and shot conversion at different game states.
At GS 0, both teams are by definition balanced in terms of TSR, as both team are at the same game state and each team’s shot created is a shot conceded by the other team. In terms of conversion, this is not a fruitful game state. This may well be due to the fact that teams are inclined to be more cautious at this score, since they have a point to lose, particularly near the end of games. A further explanation may be that this Game State, by definition, occurs more in the opening stages of matches, and teams may well be more conservative at the start of a match than they are at the end.
At GS +1, there is an interesting trade off. The TSR declines over 10% to 0.443, while the shot conversion rises to 14.1%. The tricky situation with TSR is that it works two ways. The leading team creates 44.3% of shots, but it concedes 55.7% at this game state. So, overall, the TSR of a chasing team is 26% higher (0.557/0.443) than the TSR of a team defending a single goal lead. Despite the fact that the conversion at GS -1 is better than at GS 0, teams at GS +1 convert 22% (0.141/0.116) better than teams at GS -1.
If we combine the shift in TSR and shot conversion for teams at GS -1 and GS +1, we find that a team at GS +1 pays 26% of TSR to gain 22% in shot conversion.
So, generally speaking, teams have a slightly worse chance of scoring when leading by a single goal than when chasing a single goal.
At GS +2, there is a whole new world. Teams at GS +2 have restored their TSR to 0.495, while their conversion rises further to 17.1%. Teams at GS -2 fall back in terms of TSR (to 0.505), while their conversion drops to 8.6%. Overall, both teams create a roughly equal amount of chances, but the team leading by two goals converts nearly twice as much.
In the end
Let’s turn to the opening question once more. Why are better teams able to convert a higher percentage of their shots into goals? They take a much higher proportion of their shots at favorable game states.
Who are the conversion kings of the Eredivisie? Right, PSV at 16.9%. They took 20.6% of their shots at GS +2 or higher, compared to 8.3% on average for the other Eredivisie teams…