An experienced QB brings much value to a team. A lot of that value is in the intangibles, but some of it can be directly measured.
I looked at all teams from 2007-2012 who replaced a QB with 2 or more years of significant playing time, defined as greater than 100 pass attempts per year, with a QB who did not have that amount of experience. In most, but not all, cases, this is the same as replacing a multi-year starter with a new QB.
I did not consider the school class (FR-SR) of the new QB, and because of my definition of significant experience, it does not include all possible QB replacement situations.
As measurement space I looked at the average Offensive Yards Per Game with the experienced QB and the same statistic the following year with a new QB.
Overall, what I found was that teams experienced a decline in YPG approximately 60% of the time, with an overall average decline in YPG of approximately 4%. The amount of decline varies dramatically when I consider the average YPG of the team and the number of years of experience the QB to replaced had.
This chart breaks out the changes by years of experience. Points on the graph further from the center line indicate a greater change in absolute YPG (not percent). Points above the line indicate years that the YPG increased with the new QB, points below indicate a decrease.
Of note to Nebraska fans, who will be enjoying the services of Taylor Martinez as a 4th year starter in 2013, the probability of a team which must replace a 4-year starter experiencing a decline in YPG is about 70%, with an average decline in YPG of almost 16%.
In 2012 Nebraska had an average offensive YPG of 460 which ranked 26th in the country. Assuming a similar number for 2013, Nebraska should expect a decline in YPG in 2014 of .112 (.70 x .16), or 51 YPG. This year, 409 YPG was good for 55th in the country.
When I group the average YPG into bins of 50 YPG, and don't consider the number of years of experience at QB that NU has to replace, NU falls into the 450-500 YPG bin. Looking at the data this way, NU has a 22% probability of remaining in the same YPG bin, a 28% probability of falling to 400-450 YPG, 33% probability of falling to 350-400 YPG, and an 11% probability of falling to 300-350 YPG. All is not lost, however. There is a 6% probability of actually improving to greater than 500 YPG.
(note that the 72% probability of decrease in this model is almost the same as the 70% probability of decrease in the first)
There are many factors that I didn't consider in this. For instance, I didn't consider the number of years that the replacement QB was the understudy, or how much game time the replacement had if he didn't reach the 100 pass atts threshold. All of this could impact the results.
In NU's case, however, I believe it would make the situation bleaker. It would be a difficult case to make that Coach Pelini has done a good job of getting his backup QBs significant experience. So, if this were considered for other teams, NU's backups lack of experience would likely make the probabilities of an offensive decline even greater than they already are.
What can be done? Coach Pelini must get his backups as much experience as possible. An offensive dropoff may be inevitable, but getting the backup as much real game experience as possible may make the difference between a decline of 5% and 20%.