I'd like to change the tone a bit to talk about evaluating the cornerback position. In this day and age, sites like Football Outsiders and Pro Football Focus provide fans with more statistics than they ever dreamed of seeing about what happens on the field. It is up to us, however, to decide how to use the information.
Numbers at a position like cornerback can be tricky to evaluate. Completion percentage against might seem like a good measure. A corner with a high percentage of completions against might seem to be toasted regularly. There are some flaws, though. For example, what if a corner allows an 80% rate, but only gives up 1 yard per completion? The opponent is clearly only hitting easy passes that don't do damage. He is shutting down dangerous stuff. Another area that completion percentage might be flawed is in the reality that the best guys don't get thrown against often. Back in 2009, Pro Football Focus tells us that Nnamdi Asomugha allowed a 75% completion rate. There's a catch, though. Opponents only threw at him 28 times.
More after the jump.
I think yardage allowed is a better place to start. Giving up 55 yards on a play is worse than giving up 11. If you allow 5 completions of 11 yards each, you are in the same spot as if you give up 55 on 1. IGuys like Asomugha and Darrelle Revis receive recognition for not allowing anything. There have to be some safeguards, though. A dime defensive back will see less plays than a starter and thus have less of a chance to give up yardage. Pro Football Focus does a great job by allowing you to filter stats to see only guys who participated in 25%, 50%, and 75% of their team's plays to see how the guys who spent a lot of time on the field stacked up against each other. Even this formula is imperfect, though.
As good of a measure as yardage can be, it does not give enough credit to the defender who baits throws and makes play on them. If you cover your guy and force the quarterback to look in another direction, that quarterback might find an open guy as he goes through his progression. If you bait him to throw and break up a pass, you have destroyed the entire play. It is more valuable to break up a pass. All pass break ups are not equal either. A corner might save a touchdown breaking up a long pass.
A guy like Antonio Cromartie playing across from Revis is going to give up more yardage than he otherwise might because teams will throw at him more and complete more passes since they look away from Revis. A better measure might be just to see yards allowed per play in coverage. This is a spot where completion percentage might mean something as well. Even though Cromartie allowed 645 yards in 2010, which is high, he only had a 44.6% completion rate, which shows us he was playing well. Cro got there will wildly vacillating really good and really bad performances, but he was pretty good on the whole.
This kind of mixing stats with context ultimately has to meet with the eyeball test to really evaluate corner play. If it's 3rd and 32 and your corner allows a 15 yard completion, that looks bad for both the raw yardage and completion percentage. He has done his job, though, and forced fourth down. The raw numbers also do not account for difficulty of receiver. Darrelle Revis has a tougher job faced up with Andre Johnson all game than a number two corner facing the Browns. The numbers count all the same, though. On top of this, you need to account for scheme. Revis has a tougher job following the top receiver on the other team around all game in man coverage with minimal help than does a corner in a Tampa 2 scheme who plays a lot of zone and is responsible for a small area.
There really is no silver bullet. Football Outsiders takes its best stab to control for these things with its DVOA formula, but it really is only a starting point for the discussion, not the end point. The numbers illuminate things better than they have been in the past, but they are not all encompassing. You need to know how to properly blend them.
One thing I will say is that looking at all of these factors makes you really appreciate how out of this world Darrelle Revis was in 2009. He matched up against the other team's best receiver two thirds of the time, most in the league, in a difficult man scheme and was elite in pretty much all of the numbers without adjusting for difficulty.
Let me know below how this post was. I'd like to kind of mix it up with more stuff like this to change the pace as we go through the long offseason.