The NWSL has a stats problem
I have written several times on here about statistical measures. xG is a frequent target of my skepticism, but frankly any data other than points, goals and cards (although sometimes even those) are is up for wide interpretation. Racing recently quoted that per Statsbomb, that Emily Fox led the league with 115 interceptions. The previous sentence as written is 100% true. In Bekki Morgan's profile of Emily Fox, to which I contributed, I stated “Fox was the team leader in interceptions with 46, which also put her in the top 10 in the league in that category.” While I should have quoted my source (fbref.com), that statement was also 100% true. Therein lies the problem. How many interceptions did Emily Fox have last year? The answer is “it depends” which is probably the most frustrating answer in the English language.
Opta, which is a very trusted source defines an interception as “where a player reads an opponent’s pass and intercepts the ball by moving into the line of the intended pass.” StatsBomb doesn't publish a strict definition, but says here “This is where a player reads an opponent’s pass and intercepts the ball by moving into the line of the intended pass.” Those definitions seems virtually identical in my mind, so why the discrepancy?
Here is where I move into the realm of educated guesswork. My best guess is that the key is the word “read”. Think about interceptions like this. Did the player intend to intercept (read) the pass, or just get end up with the ball? Again, I am guessing here but, I think for there to be such a wide range in the reported numbers, the 46 inceptions I reported are instances where the evaluator (we'll come back to this) determined that Emily Fox made an intentional effort to step into the path of a pass. The 115 number reported by StatsBomb surely has some “possession gained” numbers in the “interception” category. The possessions gained numbers includes (typically) tackles, aerial duels won, interceptions, blocked crosses or shield outs. It probably also includes some “ball recoveries” where a player picks up a ball that is deemed not to belong to a specific player by the evaluator. The other option which I think and hope is unlikely is that maybe one of the sources is just wrong.
All of this is to say that we as fans are at the mercy of the evaluators and the stats suppliers when it comes to numbers. My background is in math and engineering. I want to believe in data and numbers. However, I still trust my eyes more when it comes to soccer. I was originally going to write a stats review of this season, but now I don't really see the point. There is a concept in measurement system analysis that is called Gage repeatability and reproducibility (GR&R). From ASQ It is defined as the process used to evaluate a gauging instrument’s accuracy by ensuring its measurements are repeatable and reproducible. The process includes taking a series of measurements to certify that the output is the same value as the input, and that the same measurements are obtained under the same operating conditions over a set duration. Basically, for our purposes can an evaluator replicate observations under the same conditions and would a similarly trained but different evaluator come to the same conclusion as the other evaluator. The general rule in measurement system analysis, is that until the evaluators (instruments) are appropriately calibrated, the measurement system is suspect. THIS HAS BEEN MY MAJOR ARGUMENT AGAIN xG. It has now reared its ugly head in regards to basic data.
None of this is to say that you shouldn't pay attention to the numbers or statistics. You absolutely should if you are interested. I am just issuing a warning label. Racing Louisville had pretty transparent intentions when quoting the 115 league leading interceptions number for Emily Fox. Racing and I both want to put Emily Fox in the best light (although I personally believe Trinity Rodman is the Rookie of the Year). My warning is that until there is a league standard source for NWSL statistics, consider your reporting source and if your eyes told you a different story, don't necessarily dismiss your own, sometimes better, judgment.
