by Aaron Schatz

At this point, most football fans understand that tackles are a bad way to measure defensive backs. Often when a defensive back gets a tackle, he's getting it because he screwed something up in coverage. The defensive back who leads the league in tackles is probably a guy who isn't that good at his job.

Well, not necessarily, because tackles come in all different flavors. First of all, there's a difference between getting a tackle on a running play and getting a tackle on a passing play. On a passing play, there is certainly a difference between getting a sack and taking down a receiver after he catches the ball. If he catches the ball, there's a difference between getting a tackle on a guy you were supposed to cover and cleaning up after somebody else's mistake. And even if you tackle a guy you were covering, there's a difference between a tackle that shows bad coverage (say, after a 15-yard gain on first-and-10) and a tackle that shows good zone coverage (say, after an eight-yard gain on third-and-10).

For example, Morgan Burnett led all defensive backs with 121 tackles; that includes both solo tackles and assists, but not tackles on special teams. However, Burnett wasn't a particularly bad player last year, as a breakdown of those tackles will demonstrate. We can start by removing 57 tackles on run plays, fifth in the NFL, and two sacks. That leaves us with 62 tackles on pass plays, which is tied for fifth. However, 17 of those tackles prevented a gain that would pass our baseline for offensive success. Burnett ranked fourth among safeties in such plays, which we can call "pass-tackle stops." And 27 of the remaining 45 tackles were on plays our game charter either listed with another player in coverage or as either "Hole in Zone" or "Uncovered."

So I thought it might be interesting to look at which players last year racked up tackle numbers by cleaning up mistakes by his teammates, as well as screens, dumpoffs and plays where the opposing quarterback had enough time in the pocket to find the holes in zone coverage.

Here are the top 20 players from last season who had a tackle or assist on a pass that we considered an offensive success, without being listed in coverage by our game charters. I will also list the average yardage of these receptions.

Another way to look at this would be to look at just cleaning up after mistakes in coverage by removing plays marked "Uncovered" or "Hole in Zone." Here are the players who led the league in those such tackles; as you can imagine, they are all safeties.

The top tacklers in this category among cornerbacks were Atlanta nickelback Robert McClain (11), as well as Corey Webster, Jason McCourty, and Josh Wilson (nine each). The top tacklers among linebackers were Daryl Washington (10) as well as Justin Durant and Craig Robertson (nine each).

Another way to look at this is to see who makes the most tackles on pass plays overall, no matter who is in coverage or how long the reception was. Pretty much every defensive stat category in the NFL last year was led by either J.J. Watt or Luke Kuechly, and this one was no exception, as Kuechly had 69 pass tackles. He also was second in the league with 26 "pass-tackle stops," behind only his teammate Thomas Davis. Here is a list of the top defenders in pass tackles, along with how many stops they had and their pass-tackle stop rate. The league average was 33 percent for linebackers, 23 percent for cornerbacks, and 19 percent for safeties.

Davis' 32 stops in 55 pass tackles made him one of only two players last year with at least 20 pass tackles and a 50 percent pass tackle stop rate. The other was Stephen Tulloch of Detroit, at 51 percent. Here are the best and worst pass-tackle stop rates with a minimum of 20 pass tackles.

I added a few names to the bottom of the "worst pass-tackle stop rate" table for defensive backs; as you might imagine, most of the worst rates belong to free safeties, so I wanted to make sure we had a few other cornerbacks listed for those curious.

Finally, what about the opposite of the clean-up tackle? Who had the most tackles on passes where he himself was in coverage? If you remember reading the AFC playoff previews, you may be able to guess the answer. It's Cary Williams, formerly of the Baltimore Ravens and now of the Philadelphia Eagles. He had 46 such tackles last year. No other player had more than 37.

What about the cornerback coverage stats, the ones that come straight from charting and don't have anything to do with the play-by-play? We'll run the final adjusted 2012 numbers in articles next week.