In my last diary I suggested that there might be some room for extra defensive improvement due to the upgrade at DC from Greg Robinson to Greg Mattison. Although I am in general agreement with Brian that massive improvements in the defense should not be expected, I began to wonder what was possible--that is, in the past 5-6 years, has a team improved its defense by leaps and bounds? To that end, I looked at scoring defense ranks of all 120 FBS teams from 2003-2010 to see how teams improved from year to year. Based on the numbers at Rivals, here is how the data shake out:
Note: the x-axis represents changes in rank (negative is good), the y-axis number of examples (out of 840 [120 teams * 7 years]). So the distribution is more or less normal, with a change of 80 rank positions (in either direction) being the maximum, more or less. The largest improvement in our dataset is 94 positions, so if that is the maximum possible then Michigan in 2011 could move up from the 102nd scoring defense (in 2010) to 8th (in 2011). HOORAY!
I had originally suggested that this level of improvement was unlikely, but turd ferguson pointed out that my percentages were misleading, because middling- to highly-ranked defenses simply cannot improve by a large margin. Looking at teams ranked 91st or worse in scoring defense, then, we get the following chart:
You can see that teams with bad defenses improve 20 ranks on average, in part because they have more room to improve than they do to regress. 31% of the time teams ranked 91st or worse improve 30 ranks or more; and 17% of the time they improve 50 ranks or more. To get into the top quartile of defenses, a team ranked 102nd (like Michigan) needs a 70 rank (or more) improvement, which has happened 5% of the time. Looking at the teams with huge improvements, it is difficult to generalize about how they did it. Here are the most improved teams in each year for which we have data (bolded numbers represent the year in which the big improvement was made):
In some cases they seem based on the emergence of a superstar player on defense. For instance, Suh for Nebraska in 2009, which jumped from the 84th scoring defense to 2nd, or Von Miller for Texas A&M, which jumped from the 104th scoring defense in 2009 to the 27th in 2010.
In other cases you have teams that are consistently fairly good who for some reason have a collapse but then recover to their old form. UConn, for instance, usually has a pretty good scoring defense. In 2005, they were 21st, and in 2007 they were 11th in the country, but in 2006 they were 94th. Likewise, TCU has a pretty amazing scoring defense but in 2004 they were 106th in the country. The year before they were 27th, the year after they were 12th. They had some NFL talent, but all 2nd day draft picks or free agents.
Michigan is obviously not in the second type of team. Our defense hasn't been top 20 since 2006. It seems likely that for Michigan to have a good-to-great defense next year, something unexpected will have to happen. The most probable in my opinion is that one or two of our defensive players becomes dominant. Note: my excel spreadsheet is available for download here.