Which NFL Defensive Schemes Are Most Effective?

Even in such an offense-heavy era of football, there are many moments in NFL playoff history that have been defined by tide-turning defensive plays or stout stands against top-notch offenses.

We just saw Super Bowl XLIX ended by an astounding interception by undrafted rookie cornerback Malcolm Butler, and this isn’t an uncommon occurrence. In their run up to Super Bowl XLV, Packers’ defensive backs Tramon Williams, Sam Shields, and Nick Collins were instrumental in picking off Michael Vick, Matt Ryan, and Jay Cutler. Seattle Seahawks cornerback Richard Sherman saved the 2014 NFC Championship game for his team with an acrobatic deflection of a Colin Kaepernick pass.

So why do we know or acknowledge so little about these crucial players?

During the 2014 season, I did two studies on the differences in value via numberFire’s Net Expected Points (NEP) metric between certain passing and rushing schemes. I aim to do the same with defensive styles here. Over history, the look and roles of defenses have shifted, but it is still nearly impossible to win without a quality defense.

For our purposes, I’ve categorized each NFL team’s defensive scheme in two ways: defensive front and style of coverage or gap responsibility. In the following sections, we’ll see how each has performed historically in the last 15 years by NEP, which is a measure of real production on the football field. This metric assigns an expected points value to each play from scrimmage based on how it advances a team’s chances of scoring, thus giving us a better sense of how each play and player contributed to the team. What schemes have been the most successful, then?

Green Eggs and Slam: Defensive Fronts

First, let’s look at the modern base defensive fronts: the 3-4 and 4-3. The names of these schemes come literally from their alignment; the first number being the typical amount of down linemen, the second being the stand-up linebackers. The 4-3 was the next step out from the 5-2 and 6-1 formations, and the 3-4 emerged a bit later as a way to get even more quick, athletic players on the field at one time. There are various ways to align these defenses and tweak them for different purposes, but we’ll get into that later. For our purposes, also, the hybrid defenses like the Buddy Ryan “46” or Belichick hybrid package are considered 3-4 base.

Most football fans see this base defensive alignment as the primary driver for defensive difference, but is there that much difference in production? The table below shows the average Adjusted Defensive NEP (adjusted for strength of opponent) for each base defense below, as well as their production in Adjusted Defensive Passing and Rushing NEP separately. Remember, with defensive versions of NEP, the lower (or more negative) the number, the better. What do we find?

There is a slight, yet noticeable, difference between the results gained between the two base defenses. The 4-3 limits offenses by more than 11 points fewer than the 3-4 scheme in the overall Adjusted Defensive NEP. While not drastic, it is worth mentioning that the 4-3 gives up little more than a touchdown and a field goal less than its counterpart, historically. On a per-play basis, they are nearly identical, but the 4-3 still limits each phase by a minimum of 0.01 NEP per play compared to the 3-4.

Perhaps this is not the quality we should really be concerned with when examining new defenses being installed on teams. What happens when we examine the more detailed coverage and gap responsibilities?

The 500 Gaps of Bartholomew Cubbins: Defensive Styles

Now we get into the nitty-gritty of style and alignment; it’s time to go to school for a second here. I’ve broken each defensive scheme into five different styles of coverage and gap responsibility: one-gap, two-gap, Cover 2, man blitz, and zone blitz. What does all of this mean?

The teams that I define as one-gap teams have a primary focus on one-gap assignments between the offensive linemen for the defenders (defensive tackles in A gaps, defensive ends in B gaps, etc.), and they often generate pressure on the quarterback primarily through these players. Coverage for one-gap teams is primarily man-to-man cornerback pressure.

Two-gap teams focus on controlling the line of scrimmage and restricting the run game by assigning each lineman two gaps to cover. These often don’t generate as many sacks as one-gap teams, so it can be limiting for individual stats, but the goal is a united team effort. Coverage is typically man-to-man cornerback pressure.

Cover 2 teams often run a one-gap scheme up front, but it may vary. The primary delineation here is that two defensive backs sit in a two-deep shell and the middle linebacker drops into coverage often instead of blitzing or gap protecting. This is a reactionary defense with zone coverage in the secondary.

Man blitz teams tend to be one-gap fronts, but focus more on involving the linebackers heavily in bringing pressure as well as dialing up secondary blitzes as well. Coverage is, obviously, man-to-man. Buddy Ryan’s “46” defense is a typical case here, as they ask for cornerback “islands” so that safeties can be available in the box.

Zone blitz teams are almost exclusively two-gap fronts as well, to free up the linebackers to rush heavily. Dick LeBeau’s schemes fall under this look, as they prefer zone coverage but will occasionally flip linemen out to cover and bring safeties on the blitz to confuse the offense.

With all that said, how do these styles stack up against each other? The table below shows each by Adjusted Defensive NEP. Remember, the lower the number (or more negative), the better.

A couple of things become immediately clear when we examine this data.

The first is that there is a clear hierarchy here, unlike in the comparison of defensive fronts. By far, the most effective defensive style in recent years has been the Cover 2 look. Used to great effectiveness by Tony Dungy, Monte Kiffin, and others, this two-deep approach is a great “bend-don’t-break” tactic that helps to limit the deep plays and “kill shots” we’ve become so used to seeing in today’s pass-happy league. The Cover 2 is the most effective style in Adjusted Defensive NEP and Adjusted Defensive Passing NEP, more effective than the next-best scheme by a little more than a touchdown per season on average.

Interestingly, there is a different champion in Adjusted Defensive Rushing NEP. The Zone Blitz scheme has been outstanding at limiting opposing rushing games, and this shouldn’t be much of a surprise. Primarily two-gap, line-control teams like the basic Bill Parcells 3-4, the Zone Blitz focuses on controlling the admission gate at the line of scrimmage but blends in focused linebacker and safety blitzes that add additional pressure and flush out runners.

One final thought: the Buddy Ryan “46” defense that Rex Ryan, Rob Ryan, and Steve Spagnuolo run is a dinosaur. In an average season, this scheme gives up more than two touchdowns more in value than the Cover 2. Single-high safeties and solo man-to-man coverage on outside receivers is no longer the way to go in this league. I hope this artifact isn’t retired, but it needs to be revamped or else it will be about as useful as a beetle battle in a bottle with paddles on a poodle eating noodles.