by Jim Glass
Should NFL teams invest the most in offense or defense?
Previously, discussing diminishing returns to passing, I noted that marginal return analysis says investment resources - including NFL salary cap money - should be allocated among different investments to produce the largest return at the margin.
In plain English about football, this means that if your team has two all-pro wide receivers and zero competent linebackers, and you have a choice between adding a third all-pro receiver or an all-pro linebacker, you'd better go for the linebacker. Adding your first good linebacker will give more net points to your team than adding a third top receiver.
This is true even if you believe offense is more important to winning games than defense. Whatever their relative importance in the big scheme of things, you are best off making salary cap investments that produce equal returns at the margin, so that your next million cap dollars invested in offense produces the same net return as the next invested in defense. With diminishing returns to additional investments, if you overinvest in offense you will receive fewer net points than from making that last investment in defense – that third good receiver gives you fewer net points than a first good linebacker. Overinvesting in defense is the same mistake in reverse.
Moreover, underinvesting on one side of the ball can undercut, and so waste, part of your high investment in the other. If you invest so much in offense that you have a bad defense as a result, the bad positions it places your offense in will hurt the offense and offset some of the high investment in it, wasting it. If you invest so much in defense that your offense is very poor, it will likewise handicap the defense and waste some of your investment in it. Either way, your team will be worse off than if it invested on both sides of the ball evenly.
Thus says the logic of marginal return analysis, right out of any economics textbook. But is it actually true for football?
In terms of EPA, there is a simple test to make. Marginal analysis says that teams with equal-sized offensive and defensive EPA numbers should have higher total EPA than teams that have lopsided high-low offensive-defensive (or the reverse) EPA numbers. And their higher total EPA should result in a better won-lost record.
To see if this is so, I added team EPA numbers for 2001 to 2010 to a spreadsheet with W-L, points for/against data, etc., for all teams over the same period. For each team it computed offensive EPA compared to the average for the ten years, defensive EPA compared to the average, and total EPA compared to average (adding defensive EPA better than average, a negative number, as a positive one).
I then took the top 120 teams by the total EPA above average number. (Analysis is limited to this group for several reasons, the simplest being that the effect sought stands out most clearly at the top. Also, the 120 teams should be mostly the same 12 teams per year that make the playoffs, and people are generally most interested in what brings success at the top.) For each team, I computed an "EPA balance ratio" – the percentage of total EPA above average consisting of the larger of offensive or defensive EPA above average.
For instance, if a team's offensive and defensive EPA above average are exactly the same the ratio is .500, if its offensive EPA above average is two-thirds of its total EPA above average the ratio is .667, if its defensive EPA is 1.2 times its total EPA above average (its offensive EPA is below average) the ratio is 1.200, and so on.
Here are the numbers for each of: total EPA over average, EPA balance ratio, and W-L pct (by both actual W-L and Pythagorean expectation) and how each relates to the others. (Pythagorean expectation is given because it is notably more accurate than actual w-l in predicting future w-l, so it may be considered a better indicator of true team strength, but the difference is minor in these numbers).
In each table the teams are ranked by a measure and grouped in thirds (top 40, mid 40, and low 40) with the columns showing the average numbers for each group.
|Teams||EPA>AVE||W - L||Pythagorean||Balance|
|Balance||EPA > AVE||W - L||Pythagorean|
|Pythagorean||W - L||EPA > AVE||Balance|
Every way one looks at things, greater balance between offensive and defensive EPA lines up with higher total EPA and higher W-L percentage.
Here's how the relationship between total EPA and EPA balance looks graphically, with a trend line:
The teams with the most total EPA are spread out low to the right.
In the endless argument about what is more important to winning games, offense or defense, the answer in my mind is: balance.
The only 16-0 team ever, the 2007 Patriots, are famous as arguably the greatest offensive team ever. Yet even that team was more balanced than most. It is the rightmost dot on the chart, and as the eye sees, its .820 balance ratio is well below average. The fact is that of these 120 "best" teams, 54 have a balance ratio over 1.0, meaning they were below league-average strength on one side of the ball or the other. There are a lot of very unbalanced teams out there.
I won't go into what "wins championships". Basically, the sample size of playoff games is too small and the knockout format of the playoffs too artificial for the data here to prove anything about that, I think. (Although this year's Super Bowl teams, the Packers and Steelers, had balance ratios of .524 and .573.)
But my general opinion is that what wins in the regular season wins in the playoffs as well: balance helps and imbalance doesn't. Overinvestment on either side of the ball in the belief that either offense or defense wins championships is counter-productive.
A bit more evidence appeared lately indicating that the Polian-Peyton Manning Colts may have suffered from what I think of as "Coryell Syndrome"- serious overinvestment in a top offense that wins less than expected because the price of it is a too bad defense. In an offensive era with the best QB of a generation and an offense famously built around him, the team has gone only 9-10 in the playoffs.
PFR.com just produced historical playoff drive data saying that in the playoffs Peyton has been handicapped by getting the ball in the worst field position suffered by any QB in history (matched only by Warren Moon) ... and this year Peyton was eliminated from the playoffs for the fourth time while putting up solidly winning-quality QB numbers - in the last 15 years, no other QB has been unfortunate enough to lose even two such games.
In their last nine playoff seasons the Colts have averaged offensive EPA 135 points above average and defensive EPA 10 points below average. Would they have won more of those playoff games if they'd traded off some offense for a better defense that let Peyton get the ball in better field position?
I'll never know ... but have my opinion.