After thinking through the previous post on this board, the flurry of activity related to ESPN’s total quarterback rating, and further, after thinking through the notion of a meaningful 0 to 100 point stat (consider a fractional probability multiplied by 100), it hit me that with so many stats now based on an average, what is that average itself based on? If it is one season, then such a stat is only entirely meaningful for that season. If it’s more than one season, then for any particular season, that stat is not guaranteed to average to, say, 0 in the case of DVOA, or 50 in the case of ESPN’s QBR. And then it struck me, a comment from Chapter 11 of “The Hidden Game of Football“, that one reason the NFL chose the QB rankings system they did is that it is independent of the stats of other players, and that it applies regardless which season is analyzed. That isn’t true of Football Outsider’s DVOA, or ESPN’s QBR. They are relative stats and thus dependent on the definition of average used. And they only make sense and are only rationally defined for the data set over which the average is taken.

Modern relative stats are, in other words, lousy tools for comparing data from 1934 to 2004. NFL’s QBR can do that. Further issues with the “modern” stats are their complex nature, and often proprietary nature. Not only can’t they be calculated by pen and paper, the formulas are often hidden, as meaningful as the “secret formulas” in laundry detergent. If source code were published, as in Jack Dongarra’s LINPACK code, then independent verification of the formulas would be possible. That’s not possible with a proprietary code base.

Proprietary formulas strike me as a street magician’s trick, a throwback to a time when mathematicians were just beginning to understand how to solve various polynomials and so the solution techniques were held in secret. On-the-street demonstrations of problem solving skill were part and parcel of a magician’s mathemetician’s repetoire. And I don’t think we’ll see it going away anytime soon so long as people can convince others to buy books full of situationally dependent average bound proprietary stats.

Final comment: the old NFL formula is one that is linear in rates. In other words, the NFL passer rating is a linear combination of things like completion rate, yardage rate, td rate, and interception rate. Other similar formulas, stateless formulas, formulas not bound to play by play but calculable by pen and paper from a box score of games, are also in general, linear combinations of rates (often adding sack rate), and could all be generalized into the form.

Value = SUM( constant term * rate term ) + general constant.

ESPN has unveiled a new passer rating formula (see also here and here, discussion of the ratings here, here, and here), one that is complex and to be plain, not very straightforward to interpret. In the age of stats that purport to give the contribution to winning in terms of wins per season a player contributes above replacement(i.e. WARP), one really has to wonder about the value of an arbitrary 0 to 100 scale. It’s in all honesty as meaningless as the NFL’s original scale, which maxes at something less than 160.

But in order to critique the new scale at all, in anything other than emotional terms, perhaps it’s best to step back and look at some of the previous critiques of the NFL’s old formula. The one we’ll start with is Brian Burke’s 2007 critique, where he points out that TDs are a pretty arbitrary criterion, and removes them from his formula. He finally decides that the best formula he can come up with is:

QB Wins Added = (Comp% * 0.18) - (Int/Att * 50.5) - (Sack Yds/Att * 1.57) - 8

This formula has the advantage of being scaled properly. It is also simple, not as sophisticated as other formulas. How well it works is beyond the scope of this survey, but we note it for those digging for more details.

Football Outsiders uses a method called DVOA to rank quarterbacks. Again, the scale is measured in terms of “success points”, and this is abstract. But it attempts to treat the game of football as something of a state machine, using NFL play by plays as the fundamental data source, and therefore is potentially a better stat than stateless formulas. However, DVOA is a rate stat, not a cumulative stat, and there can be times when a rate stat lies to you (i.e. a high performing player who can’t stay on the field can have a very high DVOA and a very low real value to a team). Nonetheless, this is FO’s attempt to improve on the QBR.

The best and most thorough critique is also an old one, the critique of the NFL QBR by Carroll, Palmer and Thorn in the book “The Hidden Game of Football“. They devote the whole of Chapter 11 to the various formulas the NFL has used, why they were busted, and why the NFL went to the formula they do use. They then critique the formula and offer two ranking formulas of their own. We’re going to spend a lot of time on the THGF critique. To be plain, those who really want to understand it should buy the book, as used copies are cheap.

One thing to note about the Carroll et al’s historical introduction to this problem is that a stat a lot of analysts drool over, YPA, was once used as the sole criterion to judge quarterbacks. When in 1957 Tommy O’Connell won the passing trophy, it became pretty obvious that not only a rate criterion was necessary, but also a cumulative statistical component as well. YPA alone isn’t a good way to rate quarterbacks.

Original and refactored NFL ratings formulas

Later in the chapter, Carroll et al give the NFL formula as the NFL gives it to others, and then refactor the formula so that analyzing the components is easier to do. The original formula is:

RATE = 100 x [( Completion % - 30)/20 + (Average_Gain - 3)/4 + TD%/5 +
(9.5 - INT%)/4]/6

and after some mathematical gyrations, they break the formula down into the form RATE = A x [ (Completion_term + Yards + TD_term – INT_term)/attempts ] + B

and that formula is (results in the same points, but easier to conceptualize)

RATE = 100/24 * [ (Completions * 20 + yards + Tds * 80 - ints * 100)/attempts] + 50/24

Once the easier-to-understand formula is established, they begin their critique in earnest.
The critical passage is as follows:

How do you feel about giving a 20 point bonus for each completion? Not sure? Think of this. If one passer throws 2 passes and completes them both for 10 yards each, he’ll have 60 points. Another passer misses his first toss and then hits his second for 40 yards. He also has 60 points. Both passers rate the same even though the second guy moved his team twice as far!

The NFL system favors the high percentage, nickel passer. It always did, but that wasn’t nearly do obvious until lately, when several teams began to use short passes out in the flat as, in effect, running plays. If Joe Montana dumps off to Roger Craig and the play loses 5 yards, Joe still gets 15 points.

Note that the example in the first paragraph of the quote is stateful. If the example has started at the 20 yard line, then the final state of the short passer would have been a first down on the team’s 40 yard line, while the final state of the “long” passer would have been a first down on the opponent’s 40 yard line. The net expected points (see also here) from the improved field position is higher, so the second scenario should be rewarded more thoroughly. But to get that kind of evaluation requires at the least, play by play stats and to the highest level of detail, video of the game itself.

Finally, Carroll et al give two formulas they regard as superior to the NFL formula:

RATE = ( yards + TD x 10 – int X 45) / att

RATE = ( yards – sacks allowed + TD x 10 – int x 45 ) / (att + sacks)

We’re not here to analyze this formula either, but to present it to those who might be looking at ESPN’s QBR and trying to figure out alternatives.

Note: A NFL QBR calculator is here.

This is an interesting book, a beginner’s introduction to serious fan football. It’s not the easiest read and I’m not terribly fond of the first chapter, which seems to think that all serious fans are aspiring coaches. Not true. This for us is entertainment. Get past that and start looking at the overall organization of the book and the amount of information within, the discussions of positions on the football field,  the difference between a zone and man corner, the discussions of the fire zone blitz, the nice little discussion of football jargon in the back – that alone would be enough for a beginner to keep this book – and I think it sits pretty well on a football book shelf.

What it is not, as the review on Residual Prolixity points out, is a “best of” book for a reader who has read plenty of football books. Tom Gower has specific issues to complain about, and they tend to be technical in nature.  Another useful critique of this book lies in Doug Farrar’s article on FO about zone blocking. In opposition to Kirwan, who dates zone blocking to Alex Gibbs, Doug suggests that the idea of zone blocking has been around a long time, and is clearly evident in what Vince Lombardi called do-dad blocking. I own the book Doug is referring to, and the coverage of do-dad (area) blocking is extensive.

That area blocking was in common use in the early 1960s is also clear from the interview with Bob Fry in Chapter 10 of Peter Golenbock’s book on the Cowboys.

When I went with the Cowboys, we were blocking in a way that we had thrown out with the Rams two years before, because it wasn’t that good. We were still area blocking….it took a couple years before we had the back pick up the linebacker no matter what.

Take home? Kirwan isn’t the best historian. But to be fair, there is a truism in football that Amos Alonzo Stagg invented it all anyway. To the modern player, Alex Gibbs is the Bible of zone blocking. Most of the books that people like Tom Gower refer to as better are not in print, and are so old that it takes some effort to see their relevance in the modern game.

When I purchased my first copy of “Thinking Man’s Guide to Pro Football”, my father also had a football book by Dana Bible on our home bookshelf. Dana was a fine football coach, but plenty of diagrams of 6-2s and talk of “crashing ends” tended to put me off. There was probably plenty to learn from that book, but just like an old “how to” book that builds radios from vacuum tubes and 45 volt batteries, sometimes their usefulness in the modern context isn’t obvious.

This book has a few signature virtues that people seem to forget. It’s modern, it’s in print, it’s on the shelves now. And unless other people write more serious and in-depth histories of ideas in football, this kind of book will carry the day.