Does Free Agent Spending Affect NFL Defensive Performance?

The Miami Dolphins made waves when they signed Ndamukong Suh for $19 million a year last season. But did it help them?

There’s a bizarre economy in place in the United States these days.

I don’t mean capitalism, or the welfare system. I mean a more personal economy. This is an economy where people don’t look to fix their broken items, or refurbish worn and old possessions, but rather just toss it away if it’s busted and buy a new one. To solve problems, sometimes we just decide to pay first and ask questions later. Education system is busted? Throw money at it. Banking and auto industries are going under? Throw money at it.

I bet many people today would hear the legend of the little Dutch child who plugged the dam with his finger and wonder why he didn’t crumple up some Euros and wedge them into the crack.

The National Football League is no different. Front offices see flaws in their franchises all the time and decide to hang a few Benjamins off of a fishing hook and trawl it through free agency to solve their woes. Pass rush is bad, Miami Dolphins? Throw $19.1 million annually at Ndamukong Suh. Can’t keep Tom Brady off your backs, New York Jets? Darrelle Revis is willing to take $14 million a year to help you.

But does spending more in free agency actually affect NFL defenses?

Make It Rain

NFL franchises have spent an average of more than $10.4 million on defensive free agents over the past five years, hoping that these veteran presences could bring immediate results to their teams. 

Did they actually do all that much to solve teams’ defensive woes?

We can check this by using the all-encompassing Net Expected Points (NEP) metric here at numberFire, which helps us break down player value in the NFL. NEP is an analytic helps us take the numbers we get from the box score and shows how that player did versus expectation. By adding down-and-distance value to standard box score information, we can see just how much each play and each team as a whole influence the outcome of games. If a defense allows 5 yards on 3rd-and-2, it means more to the game than it does on 3rd-and-10, and those plays should be valued accordingly. For more info on NEP, check out our glossary.

So, for our study, we want to know if spending more on defense in the offseason indicated a larger positive change in defensive value from one season to the next.

We can figure this out by looking at the statistical R-correlation between these variables. Essentially, if there is a positive correlation, that means that when one variable increases or decreases, so does the other. A negative correlation means that when one increases, the other decreases, and vice-versa. The closer a correlation is to 1 or -1, the more influence the variables have on each other; the closer it is to 0, the less they are connected.

The table below shows the correlation between spending on defense in free agency and the change in schedule-adjusted Defensive NEP from the previous season to the next. In other words, how does spending impact a potential positive change in performance? Is there a strong relationship?

Adj. Def. NEP Change
Defensive FA Spending Correlation   -0.12

We do see a negative relationship in dollars to annual change in Adjusted Defensive NEP, which means that the more a team spends, the more their year-to-year value changes toward the negative (the better they are at preventing other teams from racking up NEP). This isn't an ironclad relationship and, in fact, is relatively weak. But there's an inclination towards changing for the better. The chart below depicts the scatterplot of these data points, so you can see the spread of money spent versus defensive performance, as well as a trend line that helps us see the virtual average.

Offseason Money Spent vs. Adjusted Defensive NEP Change

This isn't a good sign for free agency truthers. 

For reference, the two data points that come close to falling in line with an ideal -1.00 correlation at the extremes are the 2015 New York Jets in the upper left (who spent $36.7 million on defense in free agency and finished with an Adjusted Defensive NEP that was 103.08 lower than in 2014) and the 2013 Chicago Bears on the lower right (who spent just $7.7 million and finished with 178.39 Adjusted Defensive NEP more than in 2012). But every other season besides those two indicates this is not a relationship at all.

Mo’ Money, Mo’ Problems

The data proves it: there is very little to the idea that spending money on defense in free agency changes a team’s fortunes for the better. It won’t guarantee that they become a good defense the following year, and hardly even a better one. Even at the upper reaches of expenditure, the average change in Adjusted Defensive NEP for a team that spends at least $15 million in free agency on defensive players is just around -3.19 Adjusted Defensive NEP. That’s not a major amount of the course of a year, not to mention that a field goal less of NEP is probably not worth that much money.

The final question I have about this data is: what does it mean for fantasy football owners? Should we look to a team who spends gobs of cash in free agency to pick our team defense units for the following season? In short, it doesn’t matter. The correlation between defensive free agent spending and D/ST fantasy points is -0.07, essentially statistically irrelevant. The best strategy for picking a defense in the fantasy landscape is still to cross your fingers, throw a dart, hope for the best, and then stream them all season.

But whatever you do, don’t throw money at your defense.