DVOA FOR SPECIAL TEAMS

The problem with a system based on measuring both yardage and yardage towards a first down, of course, is what to do with plays that don't have the possibility of a first down. Special teams are an important part of football and we needed a way to add that performance to the team DVOA ranking. Our special teams metric includes five separate measurements: field goals (and extra points), net punting, punt returns, net kickoffs, and kick returns.

The foundation of most of these special teams ratings is the concept that each yard line has a different value based on how the likelihood of scoring changes with better field position. In Hidden Game, the authors suggested that the value of field position for the offense existed on a straight line with your own goal line being worth -2 points, the 50-yard line 2 points, and the opposing goal line 6 points. (-2 points isn't just the value of a safety; it also reflects the fact that when you are backed up in your own zone, you are likely going to see your drive stall, and you'll need to punt and give the ball to the other team in good field position. Thus, the defense is more likely to score next.) We use a more refined set of values based on our research, but the idea is the same.

The special teams ratings compare each kick or punt to the league average for based on the point value of field position at the position of each kick, catch, and return. We've determined a league average for how far a kick goes based on the yard line from where the kick occurs (almost always the 30-yard line for kickoffs, variable for punts) and a league average for how far a return goes based on both the yard line where the ball is caught and the distance that it traveled in the air.

The kicking or punting team is rated based on net points compared to average, taking into account both the kick and the return if there is one. Because the average return is always positive, punts that are not returnable (touchbacks, out of bounds, fair catches, and punts downed by the coverage unit) will rate higher than punts of the same distance which are returnable. (This is also true of touchbacks on kickoffs.) There are also separate individual ratings for kickers and punters that are based only on distance and whether the kick is returnable, otherwise assuming an average return in order to judge the kicker separate from the coverage. For the return team, the rating is only based on how many points the return is worth compared to average, based on the location of the catch and the distance the ball traveled in the air. Return teams are not judged on the distance of kicks, nor are they judged on kicks that cannot be returned.

Field goal kicking is measured differently. Measuring kickers by field goal percentage is a bit absurd, as it assumes that all field goals are of equal difficulty. In our metric, each field goal is compared to the average number of points scored on all field goal attempts from that distance. The value of a field goal increases as distance from the goal line increases.

Kickoffs, punts, and field goals are then adjusted based on weather and altitude. It will surprise no one to learn that it is easier to kick the ball in Denver or a dome than it is to kick the ball in Buffalo in December. Because we do not yet have enough data to tailor our adjustments specifically to each stadium, each one is assigned to one of four categories: Cold, Warm, Dome, and Denver/Mexico. An additional adjustment drops the value of field goals in Florida and raises the value of punts in San Francisco.

Once we've totaled how many points above or below average can be attributed to special teams, another formula then transforms these numbers from points to DVOA so the ratings can be added to offense and defense to get total team DVOA.

There are three aspects of special teams that don't show up in our numbers because a team has little or no influence on them -- and yet, these plays do have an impact on wins and losses. The first is the length of kickoffs by the opposing team, because no matter how strong your return man is, you can't make the other guy kick it shorter. The other two are field goals against your team, and punt distance against your team. Research shows no indication that teams can influence the accuracy or strength of field-goal kickers and punters, except for blocks. And although blocked field goals and punts are definitely skillful plays, they are so rare that they have no correlation to how well teams have played in the past or will play in the future. The numbers for kickoff length against, punt length against, and field goals against are added up on the special teams stats pages in the column marked HIDDEN.

Special teams ratings also do not include two-point conversions or onside kick attempts, which like blocks are so infrequent as to be statistically insignificant in judging future performance.

Regular season totals, playoffs not included

Revised as of 4/20/2011

This page lists an estimate of how many points, compared to league average, each team receives from the five elements of special teams: field goals/extra points, kickoffs, kick returns, punts, punt returns. The total is then converted into a DVOA percentage so that it can be added in to offense and defense to create total team DVOA.

Field goal rating compares each field goal to the league-average percentage of field goals from that distance. Yards of field position from the other four elements of special teams are translated into points using a method that gives each yard line a point value based on the average next score an NFL offense is worth from that point on the field. Kickoffs and punts are based on net yardage. Kickoff returns and punt returns are judged on return yardage only. These numbers are then adjusted for weather and altitude based on stadium type (cold, warm, dome, Denver) and week. The system is explained here. Two-point conversions and onside kicks are not included.

LAST YEAR represents last year's rank. The top number in each category is in bold type.

The final five columns represent elements not found in the standard special teams ratings.

•HIDDEN represents the advantage teams have received from elements of special teams generally out of their control: opposing field goals, kickoff distance, and punt distance. It is listed as points worth of estimated field position, and is ranked from the team with the biggest advantage to the team with the biggest disadvantage.
•WEATHER represents the estimated effect of weather, altitude, and domes on each team's special teams performance. It is listed as points worth of estimated field position, and is ranked from the team with the biggest advantage to the team with the biggest disadvantage.
•NON-ADJ VOA shows special teams DVOA without the adjustments for weather and altitude.

FootballOutsiders wrote: