In a recent post about team balance, I noted that we should expect a correlation between a team's offensive performance and its defensive performance--at least, not in the same way we would expect to see a team's passing and running performance correlate. Jared commented that although we shouldn't expect a positive correlation, perhaps we should expect a negative one.
A negative correlation between team offense and defense would mean that the better the offense, the worse the defense, and vice-versa. This would make sense in the salary cap era because the more resources a team invests on one side of the ball, the fewer resources it can invest on the other side. If you are throwing $15 million/yr at a star QB, that's $15 million less available to shore up a weak secondary. And that star left tackle drafted in the first round is a top defensive end that's going to some other team.
On the other hand, it could be that good GMs and well-run organizations would simultaneously produce teams composed of both above-average offenses and defenses. Conversely, dysfunctional organizations would produce poor squads on both sides of the ball.
It turns out that Jared was right, at least in terms of Success Rate (SR). I looked at team performance on offense and defense for all teams in the 2000 through 2009 regular seasons. In terms of SR, there was a negative correlation to the tune of -0.20.
But something strange happens when we look at things in terms of Expected Points Added (EPA). The correlation is positive, this time to the tune of 0.06, which is only marginally statistically significant for 318 cases (team-seasons).
Now look at the relationship in terms of Win Probability Added (WPA). The correlation is still positive but now significant at 0.17.
This is puzzling. It appears that the more that context is considered, the more a team's offensive and defensive performance correlate. And the less that context is considered, the more negative the relationship becomes. EPA and WPA are both much more sensitive to relatively random events, such as broken plays and turnovers, and are consequently subject to large swings.
Here's one half-baked theory. Say there is a team with a good offense and a mediocre defense in terms of SR. The good offense puts pressure on opposing offenses to respond to scoring, which allows its defense to convert a mediocre SR into better EPA, and then even better WPA. The same relationship would hold for teams with good defenses and lesser offenses. But that's just one theory, and there are bound to be better ones.
I like it when at first things make no sense. It usually means we're probably about to learn something cool.