In this paper (now forthcoming, JSE: doi: 10.1177/1527002512471538), it was shown that for every single year after the expansion to 32 teams in 2002 (until 2011), the NFL was even more competitively balanced when the strength of schedule was accounted for, without exception, using four common CB measures. Previous The Sports Economist posts on this are here and here.
Since the 2013 regular season has just been completed, we crunched the numbers on the two most recent seasons. The streak remains unbroken, once again demonstrating the importance of adjusting CB measures for unbalanced schedules.
Standard Deviation Ratio: 1.5245 (unadjusted); 1.4645 (adjusted)
Herfindahl Index of CB: 1.1453 (unadjusted); 1.1340 (adjusted)
Concentration (12) Ratio: 1.4010 (unadjusted); 1.3889 (adjusted)
Gini Coefficient: 0.2776 (unadjusted); 0.2647 (adjusted)
Standard Deviation Ratio: 1.5271 (unadjusted); 1.4396 (adjusted)
Herfindahl Index of CB: 1.1458 (unadjusted); 1.1295 (adjusted)
Concentration (12) Ratio: 1.4063 (unadjusted); 1.3904 (adjusted)
Gini Coefficient: 0.2776 (unadjusted); 0.2590 (adjusted)