Metrics that Matter: How good is Marcus Peters really?

(Metrics that Matter is a regular offseason feature that examines some aspect of fantasy through a microscope to dive into the finer details.)

Friday, the Los Angeles Rams reportedly agreed to terms with the Kansas City Chiefs to acquire 25-year-old cornerback Marcus Peters.

Peters is an interesting cornerback to analyze, because while I do think he’s probably a top-10 talent, he isn’t a cornerback we’ve ever had to fear for DFS. Last season, Peters never shadowed, and rarely ever strayed from his side of the field, spending 94 percent of his snaps lined up at left cornerback. This made things easier for opposing offensive coordinators who would just move their top receivers around the line of scrimmage to pick on Arizona’s other, more vulnerable cornerbacks. Kansas City gave up the sixth-fewest fantasy points per game to right wide receivers last year (Peters’ side of the field), but the most fantasy points per game to left wide receivers and the third-most to slot wide receivers.

On Twitter, followers began asking me what I thought Peters was worth in terms of draft picks or whether I thought he was an “elite” cornerback. It was hard for me to begin positing an answer, because, really, it’s hard to analyze a cornerback from a quantitative basis. There are countless metrics to use, each with various strengths and flaws, but I thought it’d be most fun if we created our own.

Yards per route run is the most predictive efficiency statistic for wide receivers, so it makes sense it would be about as strong for cornerbacks. However, it fails to incorporate touchdowns or interceptions, which are clearly important events to consider when analyzing cornerback’s game. So, we could look instead at a cornerback’s opposing passer rating, just as we would a quarterback’s passer rating. However, as I’ve ranted about elsewhere, passer rating overweights and double-counts completions, which have far more to do with depth of target than accuracy. Targets are also in and of themselves a negative indicator of cornerback talent – in all likelihood, the more a cornerback is targeted, the more open a wide receiver is. So, I wanted a statistic that works on a per-route basis instead of a per-target basis, and that included yards, touchdowns, and interceptions – oh, and also first downs, which are severely underrated in evaluating efficiency.

Basically, what I came up with was something similar to adjusted yards per attempt for quarterbacks, but with a few minor differences.

• It works on a per-route basis rather than a per-target or per-attempt one.
• It includes and factors in the appropriate valuation of a first down.
• It adds defensive pass interference yards into total yards allowed.

Here’s the formula:

CB Rating = ([Yards Allowed (including DPI penalty yards)] + [20 * Touchdowns Allowed] – [45 * Interceptions Caught] + [8.7 * First Downs Allowed]) / Routes in Coverage

Looking at all 72 qualifying cornerbacks (to play on at least 850 routes in coverage) over the past three seasons, here are the top-12 cornerbacks by this metric:

A few things quickly stand out.

Peters, impressively, ranks third-best by this metric. He also totals the third-most passes defensed over this stretch, which was a statistic I had considered including in our formula. Peters totals 19 interceptions throughout his three-year career, which is the most of any player over this stretch (by five interceptions), but also the fourth-most by any defender since the NFL merger. Peters also isn’t just a ball-hawk, ranking top-25 yards allowed per target. Still, as good as I think Peters is, I do think it’s a fair knock to critique him for rarely moving away from his side of the field. He typically faces a much softer level of competition than someone like Patrick Peterson, who will routinely shadow the league’s top wide receivers, and he likely statistically benefitted from much worse cornerback play surrounding him.

The Jaguars have our Nos. 1, 2, and 10 cornerbacks by this metric. The 2017 Jaguars ranked seventh-best in opposing passer rating and fifth-best in yards per pass attempt allowed this past decade. Many know how dominant they were, and especially how dominant A.J. Bouye and Jalen Ramsey were, but slot cornerback Aaron Colvin mostly has gone overlooked. Last season the Jaguars allowed the fewest yards to opposing slot wide receivers, and Colvin was a big reason why, ranking third-best in yards allowed per route from the slot in 2017 (among all 41 cornerbacks to see at least 25 slot targets). Early reports indicate he’s unlikely to be retained by Jacksonville this offseason, but the data suggest he’ll make one team very happy.

Most of the names on this list were hardly surprises, but two caught me off guard: Bobby McCain and Ross Cockrell. McCain has by far the worst TD:INT ratio of the cornerbacks on this list, but managed to make up for it in all other categories. McCain graded out 43rd-highest among all 121 qualifying cornerbacks in coverage in 2017, but graded out poorly in his first two seasons. For this reason, I’m not sure McCain didn’t just find his way onto this list thanks to small-sample variance. Cockrell too I’m skeptical about, who was traded for a seventh-round pick last summer and served as a backup for most of the season. Still, he probably should have stolen more work from Eli Apple earlier in the year, who ranked eighth-worst of 86 qualifying cornerbacks in opposing passer rating over this sample.