or: who you gonna believe? me, or your lyin’ eyes?
You all probably know that I’m kind of a stats geek. And, yes, I am; I think that we can learn a lot from breaking down individual and team metrics. I like fooling with numbers. Damn, I really AM a sad sort of stats geek.
This is to show you that, while I am a stats geek and still think it’s worth tracking statistics, it’s also important to look hard at them. Because in some cases there really are lies, damn lies, and statistics.
Specifically, I want to look hard at this statistical breakdown of goalkeeper performance put together by the usually-invaluable Chris Henderson of All-White Kit on his Twitter feed.
You’re probably familiar with the “expected goals” statistic, xG. If you’re not, there’s a couple of nice quick breakdowns here and here. Henderson has used the xG estimate and compared it to the actual goals-against to assess the keeper’s efficiency.
Unsurprisingly, keepers playing for the better teams in the NWSL, such as Bledsoe for Washington and Naeher for Chicago (and A.D. Franch for Portland) appear in the upper half of the list. They have let in fewer goals than their team’s xG in the matches they played suggested were possible, ranging from almost a goal per game for Bledsoe to about a goal every three games for Franch.
(What I find kind of fascinating on this list is that Kailen Sheridan, playing for the woeful Sky Blue, is second behind Bledsoe at almost four goals prevented every five games. Your defense sucks but You da Woman, K.S.)
As I looked over this list one anomaly stuck out at me.
Look at the stats for A.D. Franch versus Britt Eckerstrom.
Both have played three games. Franch gave up 5 goals against an xG of 6.08 for a differential of -0.36 goals/game. Eckerstrom gave up 4 goals against an xG of 1.69, for an extra 0.77 goals conceded per game.
The statistical conclusion is that Franch is nearly a goal-per-game better technically and tactically than Eckerstrom.
To a Thorns fan that seems ridiculous on its face. Eckerstrom has been solid, very solid in the three matches she’s played this season. Perhaps a trifle ahead of A.D. – who had something of a tough April and early May – perhaps a little behind, but certainly not an entire goal to the worse.
And, actually, if you break down the goals against Eckerstrom you’ll see that, yes, your subjective assessment is correct and the statistic is wrong.
The problem isn’t Eckerstrom; it’s a flaw in the xG system.
Here’s a diagram of the locations of the goals Eckerstrom has conceded in her three matches played, with the xG values for each.
Outside of the Hatch goal they’re all very low, ranging from 1.8% (DiBiassi) to 18.2% (Pressley and Brynjarsdottir). That’s because the xG system gives lower probability values to shots taken from corner kicks – which includes Pressley’s goal in Orlando and DiBiassi’s and Brynjarsdottir’s in Maryland.
It also gives lower odds for headed shots, which includes Pressley’s, and the own-goal.
It also gives very low odds to shots taken from the far outer flanks, such as a corner, which includes the DiBiassi goal.
Taken altogether, I get a total of 0.7xG for these four concessions.
Henderson’s calcs – or, rather, InStat’s, I suspect – are nearly a goal higher. I think this is because I undercounted the own-goal, which I believe in the xG system is an automatic 1.0. If so that yields Henderson’s cited xG value of 1.69.
So the problem here isn’t with Eckerstrom’s technical skill or tactical nous; it’s that the xG system dramatically underweights the attempts that resulted in goals against her.
Or, to put it another way, Eck has been desperately unlucky.
Yes, the Hatch goal was quality. The other three were a mixture of pure bad luck and defensive field player issues; Menges losing Pressley and giving up the free header in Orlando, nobody keeping Cheyna Matthews out of Eck’s grille on the DiBiassi corner, and Dagny’s freakish own-goal.
So you rock on, Eck. Henderson and I and the damn stats may keep fiddling away, but all the while you’re doing juuuuuust fine.
- PTFC v OLR: Match thread and post-match discussion - July 11, 2020
- PTFC v WASH: Match thread and post-match discussion - July 5, 2020
- PTFC v CRS: Match Thread and Post-Match Discussion - June 30, 2020