Thursday, September 3, 2015

Team Plus-Minus Page

If you direct your attention to the navigation bar you will note that there is now a new link to the team plus-minus page. I've added a brief summary below but it's a pretty simple principle. One word of caution: the data is based solely on 2015-16 data so the sample sizes are ludicrously small. That said, these numbers (particularly with penalty box touches and total shots) do stabilise relatively quickly so it's worth at least having a glance at these numbers in the coming weeks if you are worried that your initial assessment of a team is a bit off (are Chelsea really struggling? Are Leicester for real?).

Workings
Plus-minus (+/-) is a very simple metric which simply tries to adjust stats to put them in a context of a team's opponents to date. For example, we might note that two teams have each registered five shots on goal per game and conclude they are of equal ability but if one team did it against Man City and Everton while the other did it against Norwich and Sunderland we would want to adjust our expectations accordingly. As with many "advanced" stats (and I'm using that term lightly here), the goal is simply to explain a concept we are all comfortable and familiar with but often ignore when looking at numbers, hence falling into traps of being overly influenced by stats which don't tell the whole story.

The calculation itself is very simple. Let's take Palace at home for this season to date (through GW4) as a quick example. More specifically, let's look at their Shots on Target plus-minus (hSoT +/-). Their two appearances at Selhurst Park can be summarised as below:

GW2 vs ARS, 4 SoT, Arsenal concede on average 2 SoT away from home, SoT +/- would be 100% (4-2)/2
GW3 vs AVL, 6 SoT, Villa concede on average 4 SoT away from home, SOT +/- would be 50% (6-4)/4
The teams total hSoT +/- is a simple average of the games to date, so 75%

A quick word on the stats used. I've selected the four listed stats - penalty box touches, total shots, shots inside the box and shots on target - because they have shown the greatest correlation to goals scored over past seasons. Perhaps I'll do a separate post on that, though there isn't much too it. The correlation I speak of is very rudimentary and might make more learned statisticians cringe as it's closer to "draw a line of best fit in science class" than anything Nate Silver might put together but I think it does the trick to highlight the key stats.

One key to keep in mind is that while the correlation strength over a season goes from SoT, SiB, Shots and Penalty Box Touches, that's also a reverse order of the frequency with which those events happen and that can lead to wilder swings in the early part of the season. I haven't tried to pin down exactly when those numbers stabilize but I have found in the past that something like SoT can take a bit longer so exercise some caution there.

I will add the same numbers for defensive stats soon.