Benjamin Massey has basically done all of the leg work for me to write this. In this post Massey put together five seasons worth of team shooting percentages and looked at how they regressed cumulatively over time. With permission I’m going to take those numbers, and essentially repeat the methodology of this post from April 2011 where I looked the season-to-season regression of team shooting percentage in the Premiership.
In short I’m going to take a teams shooting percentage in year one, and plot it on the y axis. On the x axis is the corresponding teams shooting percentage in year 2. The resulting plot is shown below (n=56).
The amount of regression is represented by the R value of the best fit line, which in this case is ~0.26. So if we want a best guess as to the value of a teams shooting percentage in year 2, we should regress their year 1 value 74% of the way towards the league mean shooting percentage of 28.0%.
In short, a seasons worth of team shooting percentage in the MLS is ~26% skill, and ~74% luck, or one part skill to three parts luck. Does this mean that shooting has lower skill component in the MLS than the Premiership, given that I found the component due to luck in the Premiership was only 61%? Well not necessarily. In the sample Massey has used the MLS season contained fewer games than in a Premiership season (30 in 2008-10, and 34 in ’11-12, compared to 38 per Premiership season), and it makes sense that a shorter season will have a larger luck component (for example imagine how much more likely it is to flip 4 heads in a row than it is to flip 50). Is that enough to explain the difference? I’m not sure, and don’t really want to do the maths right now, but it makes sense that the gap would at least be narrowed if not overcome were the MLS season 38 games long.