The West Coast Eagles flexed their wing-muscles again this weekend, beating up a bottom-four team for the second week in a row as Melbourne got well and truly towelled up by the visitors. What was surprising was to see that Melbourne had more disposals, contested possessions and tackles than the Eagles, yet still managed to lose by 93 points. Commentators are always quick to say that a key to winning is to “win the contested ball” and “maintain possession”. But if you were to look at just the stats (excluding the score), you would be excused for thinking the contest was much closer than the score line says.
Historically, number of disposals correlates reasonably well with final margin (with some scatter). Unsurprisingly, teams that possess the ball more typically outscore their opponents.
Melbourne had more disposals than West Coast. So it seems like this game was an outlier. Did Melbourne get a lot of soft possessions in their back line? Well Melbourne only had 14 fewer inside fifties. As you might expect, inside 50s are also a very good indicator of margin.
But you can see that this still does not explain why Melbourne was beaten by so much! Having a few less inside fifties per quarter puts you at a disadvantage, but can’t explain a 93 point margin! The result makes more sense if we look at how a team’s score depends on forward-line efficiency. We would expect that, everything else being roughly equal, a team’s score is a linear function with scoring efficiency. And that is what we see:
The upper bound to a team’s score is dictated by their forward-line efficiency (which is fairly linear in the above plot). That is to say, at a fixed inside-50-to-goal conversion rate, there is a maximum score you can get because there are only so many times you can get the ball up there in a match. And the better this conversion rate is, the higher the score. The area underneath this limit is filled in by teams that function at reasonable efficiency, but just can’t get the ball into their forward line enough (i.e. are outplayed elsewhere on the ground). But Melbourne didn’t fall into that category on the weekend. The red point (Melbourne) is not sitting in the average-goals-per-inside-50-but-low-score region (where forward lines function okay, but just don’t get it enough), they are in the terribly-low-goals-per-inside-50-and-low-score region! Melbourne didn’t get smashed because their forward line was starved, they lost because their forwards couldn’t turn an inside 50 into a goal.
The numbers show that Melbourne’s game doesn’t fall into a category where West Coast were simply far too good and beat Melbourne all across the ground – Melbourne matched it with the Eagles in many areas, including disposals, tackles and clearances, and weren’t smashed in inside 50s. But still lost by 93 points. The reason that Melbourne didn’t walk away with a more-respectable 5/6 goal loss is that they had a shocking conversion rate in their forward line. I suppose the silver lining is that this probably bodes well for Melbourne once they can get Mitch Clarke, Chris Dawes and Jesse Hogan back in the forward line.