With the conclusion of week nine of the NFL season we have reached the midpoint of the 2015 regular season. All teams have played in at least eight of their sixteen games with four teams having played nine since they have not had the bye week. I decided to replicate last season’s analysis of the 2014 season to develop a “report card” for this year’s lineup of NFL team performances.
The 2014 model included three predictors of a team’s margin of victory: net yards gained per game, net turnovers per game, and net sacks per game. The first two of these also help explain variations in a team’s margin of victory in 2015, but I find no separate effect so far for sacks. Perhaps that is just because they are relatively rare events which makes their effect harder to measure across just eight or nine games. I also included a new factor this year, net yards gained or lost due to penalties. It turns out that penalty yardage has an effect on scoring that persists even after a team’s total yards gained or lost is factored into the equation. The effect is about the same for yardage due to penalties as net yards from scrimmage, about seven or eight points for every hundred yard a teams outgains its opponent.
Including those factors lets us rank the 32 teams by how well their actual performance on the field compares to the performance my little model predicts. At the top of the list is the Baltimore Ravens. They have scored an average of three points fewer than their opponents, but that is almost five points more than their statistics would predict. The next five overachievers all outscore their opponents by a margin or three or four points more than we would expect given yardage, turnovers, sacks, and penalties.
Two teams stand out as dramatic underachievers, the Tampa Bay Buccaneers and the Chicago Bears. Both teams score an average of six points fewer than their statistics would predict They are followed by the Chargers, Jets and Texans. Of those three only the Jets are predicted to outscore their opponents by a substantial margin, nine points per game, but they have managed to win by an average margin of just under five.