Last season I took a cursory look at the importance of depth at offensive line. I highlighted the thinness of the Redskins' line by pointing out that although their total salary spent on their line was in line with most other teams, a large part of their cap space was allocated to a single player, left tackle Trent Williams. Their median salary was less than half of that of the division leading teams at the time, suggesting that the Redskins annual mid-season swoon was due to a lack of replacement talent following inevitable injuries to starting linemen.
This post will go far more in depth and look at correlations between team offensive line (OL) salary and performance. Using salary data from USA Today's database for the 2000 through 2009 seasons, I calculated correlations between OL salary and various advanced offensive performance statistics. (USA Today's database ends at 2009. Additionally, the 2005 season was excluded because USA Today's database appeared incomplete that season, listing half as many players than the other seasons.)
As in my other salary analyses, I relied on cap hit as the truest measure of a player's cost to a team. NFL salary structures are notoriously complex, with base pay, bonuses, guarantees, and other factors. But cap hit comprises most if not all of those considerations, and it represents the cost to most team's most precious resource--its cap space. For each season, I adjusted all salary numbers according to the league's cap for that year to account for salary inflation.
Naturally, we'd expect that the more a team spends on its OL, the better it should do on almost all aspects of offensive performance. They would, in general, tend to both run and pass better, score more, and win more than teams that spend less on their lines. We'd also expect that teams with higher median salaries for their OL will perform better than those with lower median salaries. We'd expect that salary effects on performance would show up in both overall offensive performance and in line-specific metrics, such as sacks, QB hits, and tackles for losses.
Measuring offensive line performance statistically might be the most difficult tasks of all advanced football analysis. My approach has been to measure how little damage an opposing front-7 does (counting linebackers on pass plays only when they are pass-rushing). Still, linemen are central to all aspects of offensive play, so there should be no doubt we'd see a relationship between salary and overall offensive performance.
The results of the analysis were surprising. Most correlations between OL salary and performance were weak or non-significant, but there was an unmistakable pattern to the results. In general, higher total cap dollars allotted to the OL correlated with worse performance, while higher median salaries correlated with better performance.
The table below lists the correlations of team total OL salary and median OL salary with all relevant performance statistics. Stats preceded with a negative sign (-EPA, for example) refer to OL EPA allowed to opposing defenders.
Get the rest of this article over at Advanced NFL Stats.