This week in Sports Illustrated, Kerry Byrne of Cold Hard Football Facts makes the claim that Passer Rating Differential is the “Most Important Stat” in football. Passer Rating Differential, as Byrne calls it, is the difference between the offensive passer rating and defensive passer rating for a team. As Byrne points out, both New Orleans in 2009 and Green Bay in 2010 ranked first in this category and went on to win the Super Bowl.

Of course, we’ve long known, or at least those that have taken a look at history have known, that pass offense and pass defense are the key components in a champion. Older publications like the Hidden Game of Football discuss the importance. I’m not a big fan of passer rating for various reasons, mainly because I think the weighting is off (completion percentage is overweighted relative to its true importance). Even though I don’t rely on it at all, and was oblivious to the most self-proclaimed most important stat, I was able to also predict Green Bay as my favorite to reach the Super Bowl in 2010 as a #6 seed, because of its relative rankings in offensive and defensive net yards per pass.

Byrne tells us that 40 of 71 champions since 1940 finished 1st or 2nd in passer rating differential, and that 17 of the 25 best teams in passer rating differential since 1960 won a title. This may sound impressive, and it shows, once again, that passing is king, but it doesn’t actually make the case that the specific measure used here, Passer Rating Differential, is The Most Important Stat. It provides no relative comparison. It doesn’t show us how it differs from looking at the best teams in Adjusted Net Yards Per Attempt or Net Yards Per Attempt (and there will be a high correlation, most of those teams would appear on all lists). It doesn’t show us that the weighting is correct and optimal.

Let’s compare it to baseball and OPS. I have no doubt that you could look at the best OPS differential teams of all-time (Offensive OPS minus Defensive/Pitching OPS allowed) and that the best teams by that measure would win a lot of titles. Could I then claim, without presenting anything else, that it was the most important stat in baseball?

No, I would have to show that the derived stat–and OPS and Passer Rating are both derived or second order stats taken from combining other primary measures–was weighted properly and there wasn’t a different weighting of on-base percentage and slugging percentage that showed even better results. I would have to not only show that the best teams of all-time were good in my OPS differential, but that it was better than other things, like wOBA. My claim falls short when I don’t take that step.

I am not surprised that some measure of passing success that includes YPA results in most of the best teams winning a championship. I am skeptical that the results give us the Most Important Stat and that it can’t be improved upon by looking at the weighting. I’m going to dig into it more in the next week and look at the component rankings of the championship teams, to see where they ranked in each primary category: yards per attempt, completion percentage, touchdown percentage, and interception percentage. I’ll also look at sack percentage, which passer rating ignores but which I think is important when considering the quality of a passing game on offense and defense.

My guess is that the yards per attempt component will rank highly, and teams will be more inconsistent with where they rank at interception rate and completion percentage. I also suspect defenses that won championships will rank highly at getting to the quarterback, something for which which passer rating differential doesn’t account.

[photo via Getty]