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Statistics 101: The Defensive Backs Edition


I read that statement or something similar innumerable times last year. And when we signed Dunta Robinson, I was very skeptical. I thought then (and I still think) his return to pre-knee injury form was/is unlikely. Don't get me wrong, I certainly hope he proves me wrong. But hope doesn't always get it done ... otherwise my mom wouldn't have sold my pog collection. 

Join me after the jump where I'll tear Dunta to shreds (figurtively).

Most of the statistical mumbo jumbo outlined in this post can be found here.

The rest can be found here. 

Positive Win Probability Added

The difference between a team’s Win Probability (WP) at the start of a play and the WP at the end of the play. WPA is the measure of a play’s impact on the outcome of a game. An individual player’s WPA is the sum of the WPA of the plays in which that player was directly involved. Being directly involved is defined as an offensive player who ran, threw, or kicked the ball, was targeted by a pass, or flagged for a penalty. Defensive players are credited for WPA when they tackle or sack the ball carrier, are credited with an assisted tackle or sack, cause a fumble, defend a pass, or are flagged for a penalty.

Robinson (8 games played and 8 games started): .15

Grimes (9 games played and 9 games started): .48

Brian Williams (9 games played and 0 games started): .19

Owens (9 games played and 3 games started): .27

Franks (2 games played and 0 games started): n/a

Coleman (5 games played and 1 game started): .07

Moore (9 games played and 8 games started): .64

DeCoud (9 games played and 9 games started): .31

Schillinger (9 games played and 0 games started):  n/a


Success Rate

The proportion of plays in which a player was directly involved that would typically be considered successful. Specifically, SR is the percentage of plays resulting in positive Expected Points Added (EPA). 

Robinson: 33.3%

Grimes: 34.5%

Brian Williams: 38.9%

Owens: 25.8%

Franks: n/a

Coleman: 47.1%

Moore: 41%

DeCoud: 35.1%

Schillinger: n/a 


Passes Defended

Robinson: 3

Grimes: 8

Brian Williams: 1

Owens: 3

Franks: 1

Coleman: 0

Moore: 3

DeCoud: 2

Schillinger: 0



Robinson: 0

Grimes: 3

Brian Williams: 0

Owens: 1

Franks: 1

Coleman: 0

Moore: 3

DeCoud: 1

Schillinger: 0


Tackle Factor 

The  ratio of a player’s proportion of his team’s tackles compared to what is expected at his position. For example, middle linebackers in a 4-3 typically make 11.9% of their team's tackles. A MLB who made 12.6% of his team's tackles would have a TF of 11.9/12.6 = 1.06. TF is adjusted for a full 16-game season.

Still confused? Just take a player's tackle percentage and divide it by the player's expected tackle percentage. Tackle Factor is a better stat than Total Tackles because poor defenses are susceptible to longer drives, which ultimately produces more tackles than shorter drives.

Here are the expected tackle percentages (based on '09 tackle percentages by position and courtesy of Advanced NFL Stats):

Position 3-4 DEF 4-3 DEF
CB 9.3% 9.5%
DE 6.0% 6.6%
DT - 5.6%
ILB 11.0% -
MLB - 11.9%
NT 6.4% -
OLB 7.0% 9.9%
S 10.5% 10.0

Robinson: .69

Grimes: 1.06

Brian Williams: .56

Owens: .99

Franks: n/a

Coleman: 1.78

Moore: .86

DeCoud: .79

Schillinger: n/a


Positive Expected Points Added/Game

The measure of a defender’s impact on the outcome of his team’s games on a per-game basis. Like +EPA, +EPA/G is limited to positive plays for individual players.

PEPA/G basically breaks down a defensive player's contributions relative to the value of individual plays (example: 3 yards gained on 3rd and 1 is much more valuable than 3 yards gained on 3rd and 8). Then - by using some funky math - they convert those contributions into a points added (to the Falcon's final score) estimate. PEPA/G is explained in detail here.

Robinson: .60

Grimses: 2.13

Brian Williams: 1.08

Owens: .97

Franks: n/a

Coleman: 1.15

Moore: 3.21

DeCoud: 1.06

Schillinger: n/a


What's the bottom line?! 

(1) Robinson: Chris Houston v. 2.0 (even if he is playing with torn ligaments in his knee) 

(2) Grimes: AWESOME! 

(3) Williams: Better than I thought he'd be 

(4) Owens: Not quite there yet ... but he's working on it (and he's continuing to progress) 

(5) Franks: Undecided

(6) Coleman: He deserved to lose his starting spot 

(7) Moore: AMAZING! 

(8) DeCoud: Some room for improvement 

(9) Schillinger: Undecided 


What do y'all think? 

Next week we'll tackle LB stats, but as always: GO FORTH AND BE STATISTICAL!