The Assignment of Bye Weeks in the NFL

There are 10 NFL Teams that had a bye week, or week off, in the first six weeks of the NFL season.  If bye weeks were allocated at random by the NFL one would expect one-half of the teams to have a winning record in their first five games and one-half to have a losing record.  The Chicago Bears are the only team, however, that has a winning record through five games among these teams to receive an early bye week.  The odds of this occurring at random is about one in one hundred.

Another manifestation of this odd outcome is the excessive parity among the 22 teams that did not have a bye week before this weekend.  Eleven of these teams have three wins and three losses.  Again if bye weeks were allocated at random, only about seven of the teams would be expected to have a .500 record.

The pattern observed this year is not apparent in other NFL seasons and is most likely due to the “luck of the draw” or sampling variation.  It does not seem that the NFL deliberately allocates bye weeks early in the season to weaker teams.  If anything, interest in the NFL is likely to grow during the season so it might make more sense to have the weaker teams take their bye weeks later in the season.


NFL Replacement Referees are Less Disruptive than the 2011 Player Lockout

Work stoppages are costly to labor and management.  One cost of a strike in professional sports is the diminished quality of play that results from missed practices.  Some teams and players, especially those with less experience, are harmed more by reductions in practice time.  In an earlier blog post I demonstrated that strikes in the NBA are associated with subsequent reductions in the shooting efficiency of players.  This post compares the impact of the NFL player lockout of 2011 to the referee lockout of 2012 on NFL game outcomes.

Itis clear to NFL fans that the replacement referees, used in the first two games of 2012, make more imistakes than experienced referees.  Although the lockout with referees seems to introduce additional noise into game outcomes, it appears that referee errors are largely uncorrelated with teams’ strengths and weaknesses.  In contrast, it appears that the lockout with players favored stronger and more experienced teams.

The player lockout of 2011 drastically reduced the amount of practice time prior to the beginning of the season.  One would expect this to have a larger effect on teams with less experienced players and teams that made more roster and coaching changes prior to the season.  Successful teams are likely to have greater stability in their personnel, have more experience playing as a team, and are less likely to change offensive or defensive coaching philosophies.  Therefore it would not be surprising that a loss in pre-season practice time harmed the weaker teams more than the stronger teams.  This is exactly what happened last year.  Final score differentials in NFL games were unusually high in the first two weeks of the 2011 season, immediately after the lockout.

In contrast there is less reason to expect that the use of replacement referees during the NFL’s lockout with the more experienced referees would tend to favor stronger or weaker teams.  In fact, the final score differentials in the two weeks of the 2012 season are only slightly higher than in earlier seasons despite the use of replacement referees.

The margin of victory in an NFL game includes signals of the opposing teams’ strengths and noise due to luck, player injuries, weather, and other factors.  These factors include the impact of a shortened training camp due to the player lockout in 2011 and the use of replacement referees in 2012.  The following chart shows the mean absolute score differential for games in the first two weeks of the 2009 through 2012 seasons.The mean score differential was 26% higher early in the 2011 season than it was in the first two weeks of the 2009 and 2010 seasons.  Games were more lopsided by about 2.66 points per game in the first two weeks of 2011.  If the team most disadvantaged by the shortened preseason always lost the game, the average impact of lost practice time is a 2.66 points advantage for the winning team.  If missed practices sometimes worked to the detriment of the winning team the average gross effect on scoring would be somewhat larger.  (If the impact of reduced practice time was uncorrelated with team strength the gross impact could be as high as a swing of 5.32 points per game.)  It seems likely that shortened practices altered outcomes by at least 3 to 4 points per game and sometimes worked to the disadvantage of the team that ultimately won the game.

In contrast there is a much smaller difference between the average margin of victory/loss between the 2009 and 2010 seasons and the first week of 2012.  Although replacement referees have affected the calls on the field, their impact on the typical score differential is only 41% of the increase caused by the players lockout.

Work stoppages and strikes are costly.  The 2011 player lockout caused game outcomes to be more uneven.  Weaker teams with more rookies and roster changes seemed to be harmed more than successful veteran teams by the lockout of players and loss of practice time.  The impact of the strike may have been as high as a swing of 3 to 4 points per game, on average.  In contrast the use of replacement referees in 2012 increased final score differentials by much less.

Bounty Payments, Concussions, and the NFL

On Friday the NFL announced the findings of its investigation of the “Bounty Rule.”  The NFL stated that players and an assistant coach on the New Orleans Saints maintained an internally financed fund to reward defensive players for forcing turnovers and causing “knockouts” and “cart-offs.”  Subsequently there have been stories that the investigation will eventually lead to the Washington Redskins, where the assistant coach previously worked, and that the punishment for teams that used a “bounty” system will be severe.

This issue is a major concern for the NFL because of lawsuits over concussions suffered by former players.  There are reports that a “bounty” system will make coaches and players more likely to be held liable for the injuries suffered on the football field.  I believe that concussions and severe blows to the head are more likely on passing plays and would like to see the NFL give defenses more of an advantage for stopping the pass.  The number of severe blows to the head per season might decline if NFL teams would run more and pass less.

An alternative is to increase the punishment for defensive players after hard hits.  Some NFL fans are concerned that the game’s appeal will be diminished if more personal fouls are called on defensive players.  Although the NFL says it has stepped up enforcement of these types of fouls, NFL data reveals that the number of penalties called per game peaked in the late 1990s and is down about 8% since then.  There is also no indication that the average severity of penalty infractions have changed in the past decade.  The average yards per penalty last season was 8.4 yards, virtually identical to the average in the 2001 and 2002 seasons.

The NFL has become more dangerous because players are stronger and faster than in earlier eras.  The risk of concussions and head injuries also increased when the NFL changed its rules to “open up the game” by encouraging more passing plays.  The following chart shows that the number of passing plays per game increased after the 1978 rule limiting contact between defensive backs and wide receivers to five yards from the line of scrimmage, and has continued to increase.

NFL rule changes have made elite quarterbacks more valuable and encouraged teams to draft and retain linemen to protect the team’s most valuable player (think “The Blind Side”).  The chart also shows that teams became much better at protecting their quarterback and reduced the rate at which sacks occur by about 25% per passing play, since the early 1970s.  The total number of sacks per game has remained unchanged from the early 1970s, however, because teams pass 25% more often.

A recent study showed that running backs and quarterbacks receive the most severe blows to the head.  Although further analysis of these (and more) data is required, I suspect that quarterbacks, receivers, and running backs are most vulnerable to severe hits when they are in open space.  This is much more likely to occur on a passing play.

The NFL may be able to reduce the probability of head injuries by changing its rules to favor pass defenses.  This would reduce the number of passes and consequently the number of hits that occur when an offensive player is in the open field and more vulnerable.  First, I would allow defensive backs to maintain contact with a receiver beyond five yards.  Second, I would reduce the penalty for pass interference from 15 to 10 yards, unless it was flagrant.  In the case of a flagrant penalty the ball would be spotted at the point of the foul (if it was more than 10 yards downfield).

I would like to see the NFL consider these rule changes before attempting to further penalize defensive players for hits that are too hard and/or too close to the head.  Decisions on the field are made in a split second, and it may be difficult to provide the proper incentives for defenders to adapt on the field.  It is much easier to change the incentives faced by offensive coordinators by making it harder to complete a pass.  This may not be enough of a change to improve safety, but it might be a good first step.

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