News & Analysis

Metrics that Matter: Drafting a team defense

By Scott Barrett
May 14, 2018

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Dec 3, 2017; Glendale, AZ, USA; Los Angeles Rams defensive tackle Aaron Donald (99) against the Arizona Cardinals at University of Phoenix Stadium. Mandatory Credit: Mark J. Rebilas-USA TODAY Sports

(Metrics that Matter is a regular offseason feature that examines some aspect of fantasy through a microscope to dive into the finer details.)

Fantasy football is a difficult game, composed partly of luck and partly of skill. By playing as optimally as possible, we can always shrink the level of influence luck has on the game, but the best player still won’t always win. Especially now, with Pro Football Focus affordably giving subscribers everything they need to know in order to win their leagues – player grades, depth charts, rankings, projections, articles on theory, the industry’s best statistics, etc. – your competition might now be stronger than ever before.

In order to succeed in fantasy football, we need to try to maximize every potential edge, no matter how small. In today’s case, that means talking about an unpopular but important topic – how and when to draft a fantasy defense.

The 2017 season

Last season, in standard leagues, 12 fantasy defenses out-scored the 10th-highest-scoring wide receiver (Larry Fitzgerald). Both the Jacksonville Jaguars and Baltimore Ravens defenses actually outscored all fantasy wide receivers and tight ends last season.

While this is also partly a case against playing in standard fantasy leagues, this was something of an abnormality. Team defenses totaled the second-most fantasy points of the past decade in 2017. Jacksonville scored the most fantasy points by any individual defense this past decade, while Baltimore ranked fourth. The difference between Jacksonville and the 12th-highest-scoring defense was 4.5 fantasy points per game, or the same difference between the third-highest-scoring wide receiver (in standard leagues) and the wide receiver ranking 42nd in fantasy points per game.

Clearly, having a top scoring defense can give you a considerable edge on your opponents (and especially so in standard leagues), but it doesn’t necessarily follow that you should be going out of your way to draft a top-level defense.

Drafting a defense

There’s a prevailing narrative among fantasy experts that team defense scoring is mostly random, with sacks, turnovers, and touchdowns making up the bulk of scoring, but being highly unstable and hard to predict year-over-year. So, I decided to dig into the numbers to see whether this was true.

Basically, the narrative holds true. Sacks and turnovers (or, really, sacks plus turnovers) has the highest correlation to fantasy points for fantasy defenses, but they’re also highly unstable metrics – sacks and turnovers fluctuate wildly among defenses year over year.

The data suggest that you’re better off selecting a defense based on raw fantasy points in the year prior than any other stat we’re looking at, and even then, the correlations were exceedingly low. But it did fare much better than in-year ADP (0.03).

Of course, this would just be our starting point. We should also follow free agency and draft acquisitions on defense and look at defensive units by PFF grade. The 2018 Rams are an interesting example here. They’ve since acquired Ndamukong Suh, Aqib Talib, and Marcus Peters on defense, three players who graded out as either high-quality or elite last season. They scored 182 fantasy points last year (third-most) and feel like a good bet to beat that number next year.

Streaming a defense

As we mentioned at the top of this article, Jacksonville averaged 14.0 fantasy points per game last year, or the most by any defense this past decade. That’s highly impressive, but at the same time, Jacksonville was outscored by the average of all defenses facing Cleveland last season (14.1 fantasy points per game.) Of course, many of these defenses were likely already rostered or would have cost some FAAB money to acquire, but I think this does indicate the viability of streaming at the position.

Considering how hard it is to predict fantasy scoring at the defensive position, my preferred method in fantasy drafts (especially in leagues that draft early) is to simply not draft a fantasy defense. Rather, I’ll draft a skill-position player with upside, hoping that after the preseason games or after some preseason buzz, I might be able to trade them or slide them into my starting lineup over a less-exciting player. Or, worst-case scenario, I’ll just drop them for a defense right before Week 1. Then, throughout the rest of the season, I’ll play matchups, adding and dropping defenses until one materializes as an every-week starter.

What do I look for in a fantasy defense when streaming?

The highest week-to-week correlations I found (to team defense fantasy scoring) were in passer rating and sacks per dropbacks. In-season passer rating (for offenses) and opposing passer rating (for defenses) both had a correlation of about 0.054. When we combine these two numbers, the correlation jumps to 0.102. In-season sack rate (for offenses) sits at 0.045 and (for defenses) 0.036 but jumps to 0.075 when combining both figures. All other numbers I looked at, including pressure rate and rushing efficiency were far less highly correlated.

We should also be targeting defenses playing defenses that are heavily favored by Vegas and/or playing at home. Vegas spread had a 0.042 correlation to weekly fantasy points scored by defenses, a fairly high number in comparison to everything else we’ve looked at, but still exceedingly low overall. Since 2014, teams favored by over a touchdown average 9.5 fantasy points per game on defense (would have ranked 12th-best among all defenses last season). Teams favored by double digits averaged 10.0 fantasy points per game over this stretch (this would have ranked seventh-best). On average, over this stretch, defenses at home have outscored road defenses 7.7 to 7.0.

Beyond that, stick to intuition. Target defenses up against inexperienced or inefficient quarterbacks who are more sack-prone or turnover-prone than the average rate.

Conclusion

In conclusion, fantasy scoring for defenses is incredibly random, and more random than any other position I’ve looked at (outside of kickers, which we already discussed in-depth here).

If you’re going to draft a defense (again, I typically wouldn’t), I’d wait until the last rounds of your draft.

If you’re going to stream defenses, I’d look at a defense’s opposing passer rating in conjunction with the opposing offense’s team passer rating, the combined sack rates of both the offense and defense, and the Vegas spread. I’d then break ties by selecting whichever team is also playing at home.

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