What's Causing Parity in the NFL This Season?

Lots of experts talk about parity every week, but they seldom describe it. This article examines what parity is and talks about the reasons it occurs.

If we're going to talk about parity in the NFL, we must first define the phenomenon. Perfect parity would be a season in which all 32 teams posted an 8-8 record.

From the salary cap to the draft order, the league has several rules in place that promote parity. Do these rules actually accomplish their goals? Or are there outside forces that contribute to different teams doing well each year?

Most of our evidence of parity is anecdotal. We remember the Colts starting undefeated so many years in a row. We remember the Patriots' undefeated regular season last year. These things stick out in our minds, but are they an accurate interpretation of what's going on?

Currently, the only undefeated team left is the Tennessee Titans, and we have a feeling that they aren't going to go 16-0.

Standard Deviation of Wins

To gain an objective view of parity, I examined the standard deviation of wins since the year 2000. Standard deviation is a mathematical measure of volatility. The higher the standard deviation, the more volatility there is in the data. The higher the volatility of wins, the further we get from that perfect 8-8 season. Here's what I found:

Year Vol/Wins
2008: 1.31
2007: 3.32
2006: 3.12
2005: 3.39
2004: 3.08
2003: 3.07
2002: 2.62
2001: 3.26
2000: 3.15

As you can see, win volatility has remained in a tight range this century. The 2002 season stands out, as the league parity was greatest. In 2002, the best regular season record was 12-4, with no AFC team winning more than 11 games that year. That's parity.

Before I ran the numbers, I thought that 2007 would be where parity was noticed the least, but I was wrong. The 2005 season saw the Pittsburgh Steelers win 15 games, the Patriots 14, and the Eagles 13. These data show us how untrustworthy our memories are. We tend to remember the outlying results too often, and this skews our perception of the facts.

So far in 2008, the volatility of wins is 1.31. I fully expect this number to go up as the season progresses, but it does look like we're having a year that is closer to 2002 than 2005.

Playoff Turnover

Another way we can examine league parity is by turnover in postseason teams. If I were the commissioner of the NFL, this would be the measure I would be most concerned with. If the same teams keep making the playoffs every year, general interest in football will decrease (assuming all other factors are kept equal).

Twelve NFL teams make the playoffs each year. Since the year 2000, at least five teams that made the playoffs failed to qualify for the postseason in the following year. With almost half of the postseason teams changing over every year, it appears that parity has always existed...This year is nothing special.

What Causes Parity?

In addition to league rules designed to promote the phenomenon, there are some randomizing factors inherent in football that make it hard to enter the postseason year in and year out.

With the regular season being only 16 games, the "long run" never really has a chance to play out in the NFL. Turnovers have a tremendous effect on wins and losses, and these events are largely random.

Turnovers

While offensive-line play and quarterback experience have a large impact on turnovers, sometimes players just drop the ball. Once the ball is on the ground, luck plays a major role in who gains possession. Luck, therefore, could have a major impact on teams returning to the playoffs.

Injuries

Injuries are the greatest randomizer in football. Nobody can predict injuries; and when they happen, their effects on the probability of victory are too large to be ignored.

You'd have to be particularly obtuse to not be able to see the effect that Tom Brady's injury has had on the football season this year. This is an extreme case, but the effects of injuries are vast:

Seattle's wide receiver injuries are a large reason for their 1-4 start.

The Eagles' fortunes are tied to the health of Donovan McNabb and Brian Westbrook.

Tony Romo's pinkie injury changed the odds of Dallas making the postseason tremendously.

The list goes on.

Conclusions

From the data we've examined today, we can see that this year more teams have a chance of making the playoffs than in years past. Parity is on pace to be the greatest in this season than it has this century.

Turnovers are somewhat random and very important for winning games. We know that players will get hurt every year, but we don't know which players. Therefore, luck plays a major factor in every NFL season. These randomizing effects probably have a greater impact on league parity than all of Roger Goddell's rules combined.