## Introducing "Air Yards"

If I've proven anything over the past year of analyzing NFL stats, it's that passing is by far the most important part of the game. So let's look at the pass and passing stats in more detail than usual.

The football pass is typically judged by how many yards it produces. Quarterbacks and receivers are judged by the their yardage compiled per game or per season. But there are two distinct components of the pass.

A NEW STAT

One of the components that is often discussed is Yards After Catch (YAC). Although it is not an official NFL stat, it is kept by Stats, Inc., and has become a buzzword when talking about the west coast offense. YAC is the yardage gained by the receiver after catching the ball.

The other component of the pass, the complement of YAC, has never been mentioned to my knowledge. It has no name...until now. The distance forward of the line of scrimmage that the passer throws a complete pass through the air is hereby called Air Yards. It can be expressed in a simple equation:

Pass Yds = Air Yds + Yds after Catch

From 2002 to 2006, NFL teams accumulated an average of 3533 yds per season. Of those passing yards, 2008 yds (57%) came through the air and 1533 (43%) yds came on the ground as YAC.

THE IMPORTANCE OF AIR YARDS

Digging a little deeper, I ran a couple very simple regressions. I estimated Passing Yards per Completion using Air Yds/Completion and then using YAC/Completion. (I couldn't run a single regression using both components because multivariate regression doesn't work when the predictor variables comprise the entirety of the dependent variable).

Air Yds/Completion accounts for 56% of the variance in Yds/Attempt, while YAC/Completion accounts for 20% of the variance. As a proportion of the total variance explained by both variables, Air Yds accounts for 74%, and YAC accounts for the other 26%. Put simply, Air Yards are 3 times as important than YAC in producing total passing efficiency.

Here is one way to interpret these results. Although Air Yds yards comprise 57% of total passing yards, it is far more critical than YAC. YAC is less variable among teams. Air Yards, therefore, is much more of a "difference maker" in determining total passing yards.

In future posts, I'll examine how much QBs contribute to YAC. Air yards are a far more appropriate and accurate measure of a QB's ability than total yards or yards per attempt. We'll also see how QBs vary greatly in their ability to produce air yards. Ultimately, these results will contribute to a better QB rating formula.

### 11 Responses to “Introducing "Air Yards"”

1. Tarr says:

Brilliant! A simple, consistent way to discount the numbers put up by the David Carrs of the NFL.

There will inevitably be some people who say that this measure is unfair to WCO QBs, but my guess is that Young, Montana, et al, look about as good by this measure as they do by most others.

2. Brian Burke says:

Thanks. In the next couple posts I'll rank QBs in 2006 based on air yards. The David Carrs of the world drop to the bottom of the list. There are some surprises on the list too. Some of the "dink and dunk" passers really aren't.

The other thing I looked at was whether QBs contributed to their receiver's YAC in any significant way. The answer is they don't, and the numbers are extremely convincing.

One thing to remember about the WCO is that the short pass isn't intended to only replace the long pass. It was also intended to replace the run. I'm sure Young and Montana had solid Air Yard numbers, but unfortunately, the YAC data only goes back so far.

3. bmoore says:

Thinking back on your recent post on rushing yards, it seems likely that YAC is strongly influenced by a few catches broken for big yards. Does YAC's contribution to passing yards improve if you use the median rather than the mean?

4. Brian Burke says:

bmoore-I agree. I think median stats would be useful in any category of stat, including YAC.

I don't have access to a play by play database, which would be necessary for median stats. Otherwise I'd have to go through NFL gamebooks and hand enter all that data.

The rushing median data I received wasn't even a true median. It was a breakdown of what % of rushes were for more than X yds.

If anyone has and wants to share a play-by-play database, let me know. I'll put it to good use.

5. Domo says:

If more than one QB plays (e.g., Matt Leinart and Kurt Warner) in a game how did you differentiate YAC from air yards?

6. Brian Burke says:

Air Yards/YAC is assigned to the QB, not the team. "myway.com" nfl stats page conveniently provides YAC by QB.

7. Anonymous says:

There are a few problems with (or at least worthy questions about) how YAC are accumulated which could drastically affect this conclusion of yours:
1) Certain routes are more likely to lead to YAC, regardless of the WR (med to deep slant vs. short curl or quick sideline out, for instance).
2) Some QBs throw a much higher % of dink-and-dunk routes than others.
3) 1 & 2 could be a function of the coaching and playcalling as much as, if not more than, the QB or WR or both.
4) Whether for the above reasons or by skill, I'm sure that there would be a major difference in YAC purely by comparing the top 4-5 QBs vs. the worst 4-5 QBs.

Not that the NFL Passer Rating is the best or most important, but the top 5 in 2008 were Rivers, Pennington, Warner, Brees, and Peyton. Rodgers, Schaub, Romo, Garcia and Cassell were within 5 points of Peyton and rounded out the top 10. The 5 worst were Anderson, Fitzpatrick, Bulger, Orlovsky, and Frerotte. Only JaMarcus Russell and Tyler Thigpen were within 5 points of Frerotte (leaving about 5-6 good QBs and 9-10 mediocre and/or below average QBs).

3/5 of the top 5 were also in the top 5 in YAC, including #1 and #2. 4/5 of the top 10 by rating and 6/10 of the top 10 by rating were in the top 10 by YAC, and 8 of the top 12 by YAC had ranked in the top 10 by rating. By the same token, none of those 7 worst QBs by rating did not break the top 20 by YAC.

Does this suggest that the top 5 or 10 QBs by both standards had great receivers in terms of YAC? Not necessarily. Calvin Johnson was the 6th best WR with 20+ receptions in terms of YAC/reception. The Raiders' Johnnie Lee Higgins was #1. Devery Henderson was #2, Bernard Berrian was #3, Welker and Boldin were #4 and #5, Chansi Stuckey was #7, Arnaz Battle was 8th, Antonio Chatman was 9th, and Justin Gage was 10th...only 2/5 and 3/10 were on the same team as a top 5 or 10 QB.

YAC and rating would need to be broken down by route type in order to effectively judge the QBs and receivers.

-Mark

8. Brian Burke says:

Mark-Great points. I agree with much of what you write. In the 2 years since I wrote this, I've learned a lot, and I appreciate your analysis.

I wish I could break it down by route type, but I don't have that level of detail in the data.

One thing I would point out is that certain receivers run certain routes precisely because of their abilities. Guys without much speed but with lots of size go up high in short redzone routes for no YAC at all. Small, speedy guys line up in the slot to run a slant or screen and can gobble up lots of YAC. That's one reason why YAC appears to "travel" with the receiver more than the QB. But like I mentioned, this has a lot to do with receiver abilities.

In the final analysis, it's going to be impossible to pry apart the QB from the receiver (or linemen or coach) to assign who gets exactly what share of credit for what happens on the field. But Air Yards gives us another useful tool for understanding the passing game.

9. mikeinbrooklyn says:

I'm fairly new to the site and have been trying to follow your Air Yards/YAC analysis and the resulting QB ratings. It seems, though, the QB ratings have stopped. Is this because of the conclusion in your last paragraph in the above comment: "it's impossible to pry apart the QB from the receiver"? Have you thus abandoned your QB rating system?

10. Brian Burke says:

Hi Mike-I still think air yards is very useful, but it's not the end-all-be-all. Truthfully, it's just a matter of time constraints. I'm in kind of a pattern where I develop a concept, refine it a little, and then I move on to something new.

We'll never truly be able to separate individual performance from his teammates' purely quantitatively. We just have to do the best with what we've got.

11. Jacob says:

Hey, Brian, re-reading this for the third time, it occurred to me that there may be another factor that hides the superior value of air yards underneath the 57/43% breakdown.

The potential range for YAC runs from 1 to 99 yards. The range for air yards is contingent on arm strength and league wide maxes out at about 65 yards I think. A guy with enough speed has essentially scored, when they get behind all defenders and break away. That takes skill, of course, but the accomplishment is due to either air yards or YAC beyond the range of the last tier of defenders.

So each component of a pass is working on a distinct scale of payoff for a successful play. Just some food for thought. I'm sure if we could see those elusive median values it would have told us this and much more as well.