In the year 2012 the average NFL team gave up 231 yards a game or 3701 yards a year through the air. The average from 2009-2013 was 227 yards a game, or 3637 yards a year. I examined the stats from 2009 to 2013 and attempted to figure out how much teams passing defense deviates from year to year. Below are the passing defense numbers from 2012.
In 2013 the average team allowed 237 yards a game through the air, or 3770 throughout the year. A slight increase compared to 2012. Below are the passing defense stats for teams from 2013.
My main interest in examining these numbers was to find out what the average change in a teams pass defense was from year to year. The chart below illustrates the amount of change that teams went through from 2012 to 2013. The number does not indicate whether the change is positive in yardage or negative, but displays the total amount of change in yards. I will examine the teams that changed positively or negatively in more detail after displaying the chart.
The average change in passing yardage given up from the year 2012 to 2013 was 30 yards a game, or 480 yards in a season. The inter-quartile mean average (eliminating the top 8 and bottom 8 teams) was 26.6 yards per game or 424.8 yards in a season. The average change over the five years I examined (year to year change, ie; 2009-2010, 2011-2012 etc;) was 26.2 yards a game or 419.2 yards over a season.
The chart above displays teams overall passing defence rank. In 2012, 17 teams we're above the average passing yardage allowed in the league. In 2013, 16 teams were above the average passing yards allowed for the league. The following teams we're able to stay above average from 2012 to 2013: Seattle Seahawks, Houston Texans, Buffalo Bills, Cincinnati Bengals, Carolina Panthers, San Francisco 49ers, Pittsburgh Steelers, Baltimore Ravens, Arizona Cardinals, and the Chicago Bears. That is a total of ten teams. The following teams did not return to being above average: St. Louis Rams, New York Jets, Detroit Lions, Green Bay Packers, Kansas City Chiefs, Denver Broncos, Philadelphia Eagles. That totals to seven teams.
The following teams finished with below average defenses from 2012 to 2013: Tampa Bay Buccaneers, New England Patriots, Washington Redskins, Atlanta Falcons, Jacksonville Jaguars, Oakland Raiders, San Diego Chargers, Dallas Cowboys, Minnesota Vikings, Philadelphia Eagles. That totals to ten teams. The following teams finished below average in 2012 but improved to being above average in 2013: New Orleans Saints, Cleveland Browns, New York Giants, Tennessee Titans, Indianapolis Colts, Miami Dolphins. That is a total of six teams.
It is clearly possible for teams to improve their passing defense from one year to another. It appears that it is more likely for a team who is good one year to be good the next. More teams return to being above average then slip to being below average. The same can be true of the below average teams. More teams remain below average as opposed to improving and becoming above average.