Evaluating team performance
SoccerSTATS.com - updated on: 18 Jun 2020
In league competitions, team performance is often evaluated according to well-known indicators such as Total Points, Points per Game or Winning Percentage. To help provide more context in evaluating team results, the Relative Performance Index evaluates Points Per Game in light of an additional factor: played opponents' Points Per Game.
"It all evens out at the end of the season". We know the tune, whether it means arguing about how unlucky our team has been
in regards to points lost, suspensions, refereeing decisions, or regarding the fact that other teams have been lucky not to have
faced as many tough opponents so far.
At the end of the season, when each team has played the same number of matches, against all the top teams and all the easier oppositions, it is clear to everyone
which teams ended up having a successful campaign and which teams did not.
However when the league is half-way through, althouth the team rankings, points and points per game can be useful indicators in evaluating team performance, such values only reflect performance
within a specific context. That context is actually the opponents each team has played against, and whether each match has been played at home
or away, as discussed in Evaluating matches difficulty-level.
Overrated or underrated?
Let's say our team is ranked just one point behind their direct competitors, with a home game in hand against a lower-level team.
Are we on course? Pending a win in that game to catch up, we would think we pretty much are. Shame for those 2 points dropped last week
against that supposedly easy opponent though.
Now the league table might be telling us a story we like to hear. And with that game in hand to factor-in, if we would only be looking at
Points Per Game, instead of total points, our team would even be ranked above its direct competitors.
Yet in order to get an idea of the kind of performance our team has actually achieved compared to its competitors, we will want to look at where
and against which opponents our team's points have been picked up, and the same would apply to the competitors' points.
Evaluating the opposition
To evaluate the opposition's level, we use the same metric described on the Run-in analysis page:
an average value taking into account the home Points Per Game (PPG) of all the opponents faced away, and the away PPG of all the opponents played at home.
The resulting value is labelled as "Opponents PPG" on the Relative Performance table.
Calculating the Relative Performance Index
Example of a weighted grading system
The Relative Performance Index (RPI) can be compared to an average "grade" a student might get when taking a series of exams to validate a degree.
Depending on the nature of the studies, the exam's grading system will likely allocate more weight to grades related to assignments or subjects that are
critical to the degree's field.
For example, for a Science degree, a good grade in History will most probably not compensate for a very low grade in Mathematics, as a Science degree will likely
weight Mathematics grades higher than History ones.
Each subject or assignment will have its own weight, depending on how critical it is for the degree.
The final combined grade will then be calculated
by giving more weight to the grades or points obtained in the crucial subjects. In that example, the assumption was that, for a science degree, a good grade in Mathematics is
more meaningful than the same grade in History.
Applying weighted grades to football team results
The RPI calculation also attempts to come up with a rating that provides more weight to the team's more meaningful results. For example,
if the last two matches played by a team are (1) a home win against a low-level team, and (2) an away win at the league leaders', both results will be valued
exactly the same in the league table and in the Points Per Game calculations, yet the latter can be seen as more meaningful in terms of performance.
Each result obtained by the team will carry its own "weight" represented by the opponent PPG, and the combined opposition strength for
all played matches by a team is represented by the Opponents PPG value in the table.
To calculate the Relative Performance Index, we multiply the team's Points Per Game with the team's Opponents PPG value.
More information on on the Relative Performance Index is available on the Relative performance page.