Last week when we debuted our full DVOA ratings for 1994, one of the issues I brought up in the commentary was the growing gap between the league's worst team and best team according to DVOA. A few people wondered whether that "growing gap" was actually a mirage that came from looking at only 1994 and 2008 without looking at the years in between, so I decided to go look.
The blue diamonds on this next chart represent the difference between the best DVOA and worst DVOA each season since 1994. The red squares represent the difference between the average DVOA of the three best teams and the average DVOA of the three worst teams, just in case a season happened to have one particularly extreme team that would skew the results. Each set of data points has a trendline.
The rising gap between the best and worst teams, at least according to DVOA, appears to be a pretty clear trend. I don't know what's behind the trend. I also don't know why teams were much closer together in 2001 and 2006 than in other years. One thing you will notice, however, is that there aren't any clear "jumps" on the table during expansion years. There's a big jump from 2001 to 2002, but the trend makes sense when you look at years before 2001. The same is true for the increase when Cleveland v2.0 came into the league in 1999.