Earlier this season, I proposed an advanced soccer metric called the secondary-pass completion rate. What this measures is the likelihood that a particular player's completed pass would result in a subsequent completed pass. Since I initially presented this precursory measurement, I have made some significant adjustments and now feel comfortable claiming that it's a much better method of evaluating passing effectiveness than overall pass-completion rate.
The first iteration trended in favor of defensive players, and this happened for a couple reasons. First, defenders usually are not risk takers; they’re much more inclined to make safe and simple passes in an attempt to retain possession. With the exception of a few situations, their main responsibility isn't to attack, and the nature of their passing graphs prove that.
So, I stipulated that in order for a pass to be considered in a player's secondary pass-completion rate, both the initial and the subsequent pass must travel forward and start in the attacking half of the field. While imperfect, this does a good job at removing "defensive" passes.
The second main adjustment I made is to control for the relative difficulty of the second passes and measure against expected outcomes. Using a model that estimates the difficulty of each pass based on components such as pass location and angle, we can look at how a player's completion rate is performing against what he would have otherwise been expected to.
In other words, I'm aiming to quantify just how much easier a player is making his teammates' passes to complete. There is a subtle but important difference between this and looking at the overall pass-completion rate of the "second" player. By controlling for difficulty, we remove some of the influence that the second player's pass selection would have on the overall measurement. So, a subsequent player who is inclined to attempt (and complete) easy passes will no longer skew the secondary pass-completion rate of the initial player.
Without further ado, here are the top 10 "secondary pass" contributors in 2012. The "Percent Increase" column represents how much more likely the second player is to complete a subsequent pass after receiving a pass from the first player. For example, Jeff Larentowicz (the bona fide 2012 MLS leader in secondary pass completion) increases his teammate's pass-completion rate by a whopping 7.5 percent.
An important caveat to recognize is that this is not a predictive metric and it's very much dependent upon the particular system that each player played in during the season. For example, when Benny Feilhaber moves into the Sporting Kansas City midfield in 2013, there are no guarantees that this system will be conducive to his impressive secondary pass-completion rate.
Similarly, if Real Salt Lake decide to play a completely different brand of soccer next year, I'd expect that Kyle Beckerman would drop rather quickly out of the top 10.
While secondary pass-completion may be good at looking back at a player's past contribution to his team's system, it does not solve the front-office critical problem of quantifying a prospective player's "fit" into another system. For this, we must still rely upon much more qualitative methods.
For now, anyway.