It has all kind of characteristics to make it both in the wide world of football blogging, and in the even wider world of football journalism. The Total Shots Rate, or TSR, is simple and easy to explain and it requires little data. Yet so far it is the single most powerful predictor of future performance of football teams.
For those not yet aware of the concept, let me explain shortly. TSR is simply the fraction of shots created by a football team in a single match, or over multiple matches. If Feyenoord creates 10 shots against Ajax, while Ajax creates 20 shots in that same match, Feyenoord’s TSR will be 0.333 and Ajax’ TSR will be 0.667. The total TSR over a single match will always be 1 and since two teams divide that total, average TSR’s of all teams in a league will always be 0.500. Over multiple matches, simply add together the number of shots created by your team and divide by the total number of shots in those matches.
But despite being the most powerful predictor around, TSR has it’s disadvantages too, with the most obvious one being that it does not correct for strength of schedule. The best teams in a league generally have a TSR of around 0.700, while at the lower end of the table TSR’s of 0.350 are more common. So the better teams seem to be around twice as good as the weaker teams with respect to generating shots. This leads to considerable bias throughout the season, as teams experience a different spread in strength of the opponents they face, but at the end of the season, when all teams have played each other twice, most of this bias has disappeared. The only bias remaining comes from the fact that teams don’t play against themselves, so the best team does not play the best team, while the weakest team does not play the weakest team. So better teams face on average lower TSR opposition compared to weaker teams.
At 11tegen11, we’ve introduced a model to predict the final standing of the Eredivisie table based on TSR. Since shots are nearly ten times more frequent than goals, the model identifies better teams much faster than the regular league table does. The main problem with the model is that teams have different strengths of schedule. After fourteen matches have been played, PSV tops the Eredivisie table in terms of TSR with 0.730. But before the first half of the season is over and all teams have faced each other once, they still have to play Ajax (0.560), Twente (0.559) and N.E.C. (0.498). So it’s safe to assume that their TSR of 0.730 will fall over the coming three matches. Meanwhile, Heerenveen (0.469) still has to play Willem II (0.385), Roda (0.353) and Utrecht (0.544). So Heerenveen’s TSR will likely be an underestimation of their true strength.
In order to tackle this problem, we will introduce the ‘Relative Shots Rate’, or RSR. The RSR is computed by comparing the number of shots created by a team against the average number of shots created by all teams in the league against that same team. This compares the performance of a team in a certain fixture with how all clubs have performed in that same fixture. Thereby correcting for the strength of schedule.
So, Ajax concedes on average 8.0 shots when playing at home, and VVV created 4 shots in Amsterdam. This gives VVV’s offense a -4.0 for that match. Meanwhile, VVV concedes 17 shots in that same match, against a league average of 13.6, so VVV’s defense record for that match is -3.4. If you would do this for every match played, and then add a team’s offense en defense record separately, you get an overall offense and defense performance that represents the average amount of shots that a team creates or conceded compared to league average.
Over fourteen matches, PSV has created on average 5.95 shots more than league average, while they have conceded 3.59 shots less. Now, these numbers can be converted to a single parameter that we will call the ‘Relative Shots Rate’, or RSR.
Computing the RSR
The average number of shots in a 2012/13 Eredivisie match has been 12.71. So, against the average opponent, one can expect PSV to create 12.71 + 5.95 = 18.66 shots and PSV can be expected to concede 12.71 – 3.59 = 9.12 shots. So the best estimate for shots when PSV plays an average league opponent would be 18.66 shots created by PSV and 9.12 shots conceded by PSV. This translates into a RSR for PSV of 18.66 / (18.66 + 9.12) = 0.672.
Now, we’ve learned that PSV’s TSR of 0.730 is quite a lot higher than their RSR of 0.672. This should indicate a strong series of fixtures coming up before the season is at its half-way stage. And indeed, with Ajax (RSR 0.554), Twente (RSR 0.538) and N.E.C. (RSR 0.486) still to play, PSV likely won’t maintain their TSR as high as 0.730.
As mentioned above, Heerenveen have three relatively easy fixtures coming up before the half-way stage of the season, playing Willem II (RSR 0.402), Roda (RSR 0.393) and Utrecht (RSR 0.536). So Heerenveen’s TSR of 0.469 is likely to be a slight underestimation of their strength.
In the end
So, while TSR is more straightforward and easier to explain, RSR offers a better representation of a team’s strength. It eliminates the bias of strength of schedule, and also allows to correct for situations where teams have played more home or away matches. On top of that, it is possible to create separate RSR’s for home and away matches, but we will save that for a later post…
Shot data provided by Infostrada Sports.