Behind the DVOA ratings
by Aaron Schatz, Special to FOXSports.com
Off-season reviews:
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Conventional NFL rankings measure teams by adding up all their yards no matter what situations they came in or how many plays it took to get them. Now why would they do that? Football has one objective — to get to the end zone — and two ways to achieve that, by gaining yards and getting first downs. These two goals need to be balanced to determine a player's value or a team's performance. All the yards in the world aren't useful if they all come in eight-yard chunks on third-and-tens.
What about measuring teams by points scored and points allowed? Well, that's a better measure — more accurate, over the long term, than wins and losses — but it won't tell you why a team scored or allowed those points. Often a mediocre offense will score lots of points if the same team has a strong defense, because the offense is always starting in good field position. (The 2004 Buffalo Bills, for example.)
Then there's the question of schedule strength. It is certainly easier to score against Green Bay than it is to score against Chicago. By adjusting ratings for the quality of the opponents, we can get a better picture of how good the offenses and defenses really are.
Special teams are included in our ratings too, by judging punts and kickoffs against the league average based on where the ball is kicked, where the return starts, and where it ends. Field goals are judged against the league average from each distance. And because it is easier to kick in Denver or a dome, and harder in Buffalo in December, all special teams numbers are adjusted based on weather and altitude.
(For those curious, these are the main differences between DVOA and the Aikman Ratings also featured on FOXSports.com: opponent adjustment and special teams. Also, DVOA has more jokes.)
Does it work? Absolutely. The only stat that is a more accurate indicator of how many wins a team will have is points scored and allowed. And DVOA is more consistent from year to year than points or yards. Over the past few years, DVOA has been a better predictor than wins or points when it comes to figuring out how many games a team will win the following year, and the same is true when it comes to using first half stats to predict wins over the second half of the season.
But DVOA isn't only valuable as a single rating to rank teams from 1 to 32. Because it is based on individual plays rather than points or wins, it can be broken down to analyze each team's strengths and weaknesses. DVOA can be separated not only by player but also by down, or by week, or by distance needed for first down. This can give us a better idea of not just which team is better but why, and what a team has to do in order to improve itself in the future.
The list of top DVOA offenses on third down, for example, is more accurate than the conventional NFL conversion statistic because it takes into account that converting third-and-long is more difficult than converting third-and-short, and that a turnover is worse than an incomplete pass because it doesn't provide the opportunity to move the other team back with a punt on fourth down.
These ratings use a weighted DVOA which discounts games early in the season in order to provide a more accurate look at how good teams are right now. Games played more than eight weeks ago become gradually less important.
The ratings table gives each team's DVOA rank from 1-32 in offense, defense, and special teams. When DVOA is used in commentary, it is important to note that positive numbers represent more offense — therefore, defensive ratings get better the more they drop below zero, and a positive percentage means a below-average defense.
Total DVOA ends up as offense minus defense plus special teams. The best team in the league will usually be about 40 percent on the year, the worst team around -40 percent.
You'll find more on the original statistics from FootballOutsiders.com on this page.
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