Brandon Lee Gowton recently wrote an excellent "lukewarm take" on the strengths and weaknesses of the Eagles' front line. This paragraph jumped out at me, though:

PFF had them ranked eighth in pass protection and first overall in run blocking. Football Outsiders ranked them ninth in pass protection and 29th in run blocking.

One says they're terrible at road grading; the other says 'best in the NFL.' How could they see it so differently? You would think that two groups attempting to carefully rate the front line would come up with numbers that are roughly similar, not first and (nearly) worst.

Granted, it's hard to measure the success of an offensive line in any way other than rushing yards. But if you do, the quality of the running back can't be removed from the equation. It's clear that both Dallas and Philadelphia have excellent lines, but last year they also had the first and third best running backs in the league, respectively, by yards gained.

For 2015, Philadelphia at least maintained that quality at RB by signing away league-leading runner DeMarco Murray. Dallas? Well, it almost seems like Jerry Jones got into an argument in a bar, where he was bragging about how great his line was, and somebody said, "Oh yeah? I bet you wouldn't gain all those yards if, uh, Darren McFadden was your running back." And Jerruh said, "The hell I wouldn't. I will show you, smart mouth."

Unless Dallas trades for Adrian Peterson (or signs Ray Rice, who fits the Cowboys' new Bad Boys image even better), fans should get a pretty good test of just how important having an elite back was to the success of that OL.

None of which explains the PFF/Football Outsiders discrepancy, since both teams had excellent running backs last year. The difference has to do with the very different methodology used by the two sites, and it has some interesting implications for the Eagles.

Pro Football Focus has analysts watch and grade each play, assigning scores between +2.0 and -2.0 to each player on each play. This is inherently subjective, despite their attempts to define clear criteria for analysts to use, and it's not hard to find seemingly crazy examples of "bad" scoring. At the same time, they try to include the context of a situation in their grade.

So, having watched tape of all of the Eagles 2014 games, the graders at PFF were very impressed with Philadelphia's run blocking, despite all the injuries on the line. Only 11 of the NFL's 32 teams had positive grades at all.

Dallas was second with 55.7 points, more than a dozen better than #3 San Francisco. Yet the Eagles blew them out of the water with 85.7 points, nearly double even the third place 49ers. Only four teams in the league had 30 points total for the whole year; Philadelphia had 30 more than the next best team.

At Football Outsiders, on the other hand, they try to use more objective statistics. For the offensive line, for example, they rank teams by a statistic they call "Adjusted Line Yards," which assigns different values to yardage depending on how short or long a run is. Losses are punished especially harshly.

Based on regression analysis, the Adjusted Line Yards formula takes all running back carries and assigns responsibility to the offensive line based on the following percentages: Losses: 120% value

0-4 Yards: 100% value

5-10 Yards: 50% value

11+ Yards: 0% value

In other words, they rate the OL entirely on the success of the running back, and penalize heavily for losses, while minimizing the line's credit for longer runs.

This is a formula almost perfectly designed to penalize the combination of LeSean McCoy and Chip Kelly's offense. Kelly's system uses an athletic if slightly undersized OL and wide receiver blocking to spring backs for longer runs at the second and third level.

Because McCoy runs so evasively, he is often tackled for a loss, which has hurt his statistics as well as frustrating coach Kelly. In 2013 and 2014, he led the NFL in runs of zero or fewer yards with 125. At the same time, he gained almost half his ground last year (619 yards) on runs of 10 yards or more.

The good news is that swapping Murray (and Ryan "One T" Mathews) for McCoy should eliminate both these losses and the statistical discrepancy. Football Outsiders has a statistic for RBs called "success rate," which measures their consistency by their percentage of successful runs. While McCoy was 3rd in the NFL for yardage, he was 22nd in success rate (at 45%). DeMarco Murray was 3rd, at 54%, ahead of even Marshawn Lynch.

In other words, the weakness of the Eagles front line in FO's statistics was specific to LeSean McCoy. With Murray and Mathews, both services should rate the Eagles run blocking highly. And the offense should also run better, especially in the red zone.