The teams (62 total) that returned an experienced quarterback (at least 100 pass attempts the previous season) and had him play significantly (at least 100 pass attempts this season) in 2006 went a collective 238-248 in conference play in 2005. This is a winning percentage of .490 and equates to just under a 4-4 record (3.91-4.09) in a standard 8 game conference season. In 2006, those same teams improved to 269-223 in conference play. This is a winning percentage of .547 and equates to a 4.37-3.63 record in a standard 8 game conference season. This is an improvement of roughly 1/2 game in the conference standings.
The teams (53 total) that did not return an experienced quarterback in 2006 went a collective 214-204 in conference play in 2005. This is a winning percentage of .512 and equates to a conference record of 4.10-3.90 in a standard 8 game conference season. In 2006, those same teams regressed to 188-234 in conference play. This is a winning percentage of .445 and equates to a conference record of 3.56-4.44 in a standard 8 game conference season. This is a regression of a little more than 1/2 game in the conference standings.
21 teams (33.9%) that returned an experienced quarterback improved by at least 2 games in the conference standings. 8 teams (12.9%) improved by at least 3 games in the conference standings. 13 teams (21%) declined by at least 2 games in the conference standings. 5 teams (8.1%) declined by at least 3 games in the conference standings.
10 teams (18.9%) that did not return an experienced quarterback improved by at least 2 games in the conference standings. 5 teams (9.4%) improved by at least 3 games in the conference standings. 21 teams (39.6%) declined by at least 2 games in the conference standings. 10 teams (18.9%) declined by at least 3 games in the conference standings.
Those are the results. Dissect them at your leisure. I will say this, it appears that it may not be as valuable to return your starting quarterback (12.9% that returned theirs improved by at least 3 games and 9.4% that did not improved by at least 3 games) as it is damaging to have him leave (more than double the chance--18.9% to 8.1% of declining by at least 3 games). As promised, in a few more days, I'll look at how BCS and non-BCS schools compare when they return and lose their quarterback.