FanPost

How can we look at our defensive backs objectively?

Too many people are throwing around PFF grades without really understanding that they're subjective grades given out by some dude.

Historically, its been pretty hard to rate individual defensive players. But I think I have a way:

Here's how I'm looking at the data, and I think its a superior way to look at individual defensive players. I'm taking +EPA, which is an excellent objective statistic -- not like PFF grades -- and adjusting it for playing time (+EPA goes up with playing time, naturally),


Here is +EPA per snap:

Player EPAplus per snap
21-E.Wright 0.057
29-C.Culliver 0.043
22-C.Rogers 0.032
20-P.Cox 0.031
25-T.Brown 0.027
24-N.Asomugha 0.022

Positive Expected Points Added (+EPA) -- The Expected Points Added attributed to a defensive player, limited to only the plays that are net positives for his team. It is a measure of a defender’s impact on the game score in terms of play-making ability. Like for +WPA, only positive plays are considered for individual defenders because very good individual defensive plays can still result in net losses in EPA. However, overall individual EPA likely correlates well with +EPA.

Here's what it look like if I look at 'big plays' - +WPA (win probability added) per snap.

Player WPAplus per snap
21-E.Wright 0.00180
20-P.Cox 0.00125
29-C.Culliver 0.00097
24-N.Asomugha 0.00077
22-C.Rogers 0.00065
25-T.Brown 0.00056


Positive Win Probability Added (+WPA)
– The Win Probability Added attributed to a defensive player, limited to only the plays that are net positives for his team. It is a measure of a defender’s impact on the outcome of games in terms of play-making ability. Only positive plays are considered for individual defenders because very good individual plays can still result in net losses in WPA. For example, a safety who makes a shoe-string tackle to stop a TD would be a great individual play, but the play as a whole would still be a net negative outcome for his team. However, overall individual WPA likely correlates well with +WPA.

Here they are multiplied together:

Player Combined (x*y)
21-E.Wright 0.0001024
29-C.Culliver 0.0000417
20-P.Cox 0.0000383
22-C.Rogers 0.0000207
24-N.Asomugha 0.0000167
25-T.Brown 0.0000151

Assessment: First, we have a pretty deep secondary. Second, Eric Wright is a steal. Third, How can Carlos Rogers salary be justified, especially if Nnamdi has a decent season?

Sources:

http://wp.advancednflstats.com/defenderstats.php?year=2012&pos=CB&season=all

http://www.footballoutsiders.com/stats/snapcounts

http://www.advancednflstats.com/2010/08/glossary.html

This is a FanPost and does not necessarily reflect the views of Niners Nation's writers or editors. It does reflect the views of this particular fan though, which is as important as the views of Niners Nation's writers or editors.

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