posted recently that plot each team's defensive performance against its offensive performance. Each graph uses a different metric: Success Rate (SR), Expected Points Added (EPA), and Win Probability Added (WPA). Instead of looking at the connection between offensive and defensive performance, look at the shape of the pattern formed by the data. Notice that the width of each plot is bigger than the height. In other words, offenses have wider distributions than defenses in SR, EPA, and WPA. The data set is comprises all 2000 through 2009 regular season teams.
In terms of SR and EPA, the best offenses are "better" than the best defenses in terms of performance. And in terms of WPA, the best offenses have bigger impacts on game outcomes than the best defenses do. This is something I wrote about three years ago, when I noticed that the distribution of yards-per-play efficiency was wider for offenses than for defenses. Now, in terms of more advanced statistical measures in a broader set of data, the same trend holds.
A variable's standard deviation (SD) is a measure of the width of its distribution. The ratio of the SD of offensive SR to the SD of defensive SR is 1.25. The ratio for EPA is 1.27. And the ratio for WPA is 1.26. Offenses are spread out 25% wider than defenses in terms of performance and impact on outcomes.
The reason, I suspect, is that most of the offense flows throw the QB or RB. The QB in particular is singularly critical to offensive success. The QB is responsible for much more than just throwing passes. He calls audibles, reads the defense, calls blocking assignments, and is responsible for organizing and managing the offense. Nearly every play an offense makes is heavily dependent on the skill of a single player.
On defense, success depends on a more equal division of responsibility. The wider the division of responsibility, the more "average" the success of the squad. In individual-player sports such as tennis or golf, individuals can dominate the field for many years. Federer or Woods did not have their talent level diluted by less dominant teammates. A five-man bball team can be "star-based" because there are only four other players to dilute the skill of the star.
But football teams average their talent level over at least 22 starters. The QB, and perhaps to some degree the RB, are the exceptions. Because offensive players do not make equal impacts on the outcomes of plays, the effective talent level is less diluted. The result is that the distribution of offensive success is wider than that of defensive success. The best offenses therefore tend to be better statistically than the best defenses.