One exception to the use of DVOA/DYAR, and the use of "play success" instead of raw yardage, is the rating system for offensive and defensive lines. Actually, these are only measures of running plays, and of course the defensive numbers don't measure just the defensive line, but the whole front seven against the run.
One of the most difficult goals of statistical analysis in football is somehow isolating how much responsibility for a play lies with each of the 22 men on the field. Nowhere is this as obvious as the running game, where one player runs while up to nine other players -- including wideouts, tight ends, and fullback -- block in different directions. None of the statistics we use for measuring rushing -- yards, touchdowns, yards per carry -- differentiate between the contribution of the running back and the contribution of the offensive line. Neither do our advanced metrics DVOA and DYAR.
We have enough data amassed that we can try to separate the effect that the running back has on a particular play from the effect of the offensive line (and other offensive blockers) and the effect of the defense. A team might have two running backs in its stable: RB A, who averages 3.0 yards per carry, and RB B, who averages 3.5 yards per carry. Who is the better back? Imagine that RB A doesn't just average 3.0 yards per carry, but gets exactly 3 yards on every single carry, while RB B has a highly variable yardage output: sometimes 5 yards, sometimes -2 yards, sometimes 20 yards. The difference in variability between the runners can be exploited to not only determine the difference between the runners, but the effect the offensive line has on every running play.
We know that at some point in every long running play, the running back has gotten past all of his offensive line blocks. From here on, the rest of the play is dependent on the runner's own speed and elusiveness, combined with the speed and tackling ability of the defensive players. If Frank Gore breaks through the line for 50 yards, avoiding tacklers all the way to the goal line, his offensive line has done a great job -- but they aren't responsible for most of that run. How much are they responsible for?
For each running back carry, we calculated the probability that the back involved would run for the specific yardage on that play, based on that back's average yardage per carry and the variability of their yardage on every play. We also calculated the probability that the offense would get the yardage based on the team's rushing average and variability without the back involved in the play, and the probability that the defense would give up the specific amount of yardage based on its average rushing yards allowed per carry and variability. For example, based on his rushing average and variability, the probability in 2004 that Tiki Barber would have a positive carry was 80% while the probability that Giants would have a positive carry without Barber running was only 73%.
Yardage ends up falling into roughly the following combinations: Losses, 0-4 yards, 5-10 yards, and 11+ yards. In general, the offensive line is 20% more responsible for lost yardage than it is for yardage gained up to four yards, but 50% less responsible for yardage gained from 5-10 yards, and not responsible for yardage past that. Thus, the creation of Adjusted Line Yards.
Adjusted Line Yards take every carry by a running back and apply those percentages. (We don’t include carries by receivers, which are usually based on deception rather than straight blocking, or carries by quarterbacks, which are almost always busted passing plays unless they involve someone like Colin Kaepernick or Cam Newton.) Those numbers are then adjusted based on down, distance, situation, opponent and whether or not a team is in the shotgun. (Because defenses are generally playing pass when the quarterback is in shotgun, the average running back carry from shotgun last year gained 5.36 yards, compared to just 4.16 yards on other carries.) The adjusted numbers are then normalized so that the league average for Adjusted Line Yards per carry is the same as the league average for RB yards per carry (in 2013, 4.10 yards).
Runs are listed by the NFL in seven different directions: left/right end, left/right tackle, left/right guard, and middle. Further research showed no statistically significant difference between how well a team performed on runs listed middle, left guard, and right guard, so we also list runs separated into five different directions. Note that there may not be a statistically significant difference between right tackle and middle/guard either, but until we can research further (and for the sake of symmetry) we do still split out runs behind the right tackle separately.
These splits allow us to evaluate subsections of a team’s offensive line, but not necessarily individual linemen, as we can’t account for blocking assignments. We don't know when a guard is pulling and when a guard is blocking straight ahead. We know that some runners are just inherently better going up the middle, and some are better going side to side, and we can't measure how much that impacts these numbers. We have no way of knowing the blocking contribution made by fullbacks, tight ends, or wide receivers.
Other numbers we use to measure the running game:
- Open Field Yards gives the portion of the team's rushing average gained after the first 10 yards of each run. So for a 10-yard run, no yards are counted; for a 15-yard run, five yards are counted; for an 80-yard run, 70 yards are counted. This number gives you an idea of how much of a team's running game was based on the breakaway speed of the running backs. A team with a low ranking in Adjusted Line Yards but a high ranking in Open Field Yards is heavily dependent on its running back breaking long runs to make the running game work. This number is not adjusted in any way.
- Second Level Yards gives the portion of the team's rushing average gained 5-10 yards past the line of scrimmage. So for a five-yard run, no yards are counted; for a 10-yard run, five yards are counted; for an 80-yard run, still only five yards are counted. This number represents the midpoint between the contributions of a team's offensive line and its running back. A low ranking in Open Field Yards coupled with a high ranking in Second Level Yards indicates that the team's offensive line is opening holes, but its backs aren't proficient open-field runners. This number is not adjusted in any way.
- Power success measures the success of specific running plays rather than the distance. This number represents how often a running attempt on third or fourth down, with two yards or less to go, achieved a first down or touchdown. Since quarterback sneaks, unlike scrambles, are heavily dependent on the offensive line, this percentage does include runs by all players, not just running backs. This is the only stat given that includes quarterback runs. It is not adjusted based on game situation or opponent.
- Stuffed measures the percentage of runs that are stopped at or before the line of scrimmage.