From week 1 to Week 17, here are the ratings:
80, 99, 82, 93, 102, 70, 76, 63, 84, 47, 94, 57, 66, 83, 63, 48, 94.
( i.e. Week 1= average 80 passer rating, Week 2= 100 etc..)
You can slice and dice these numbers anyway you like, but obvly you have to be careful and consistent in not doing any curve fitting. Whatever tranches you use, there is clearly a rolling moving average that TRENDS DOWN FROM THE BEGINNING TO THE END OF THE SEASON. Essentially, Manning peaks in his first 5 games and then slides thereafter, leveling off after Week 10. Remember that this data has 56 observations, a fairly decent set which satisfies my criteria (should be between 30 to 100 datapoints) for relevance.
Average Weeks 1-5: 91
Average Weeks 6-10: 68
Average Weeks 11-17: 71
A dropoff of 21 points in the passer rating is significant, especially when considering this is based on a total of 56 games spread out over 3-4 seasons. I'd rather have the next 8 datapoints for 2008 before using this, but keep in mind that if you just use 2005-2007 you get the same results (yes, the dropoff is actually a little worse).
Why the dropoff?
Lack of urgency? Tiring? Defenses catching up to Manning's/offensive tendencies? Maybe after Manning gets out of the box really well in each season, the coaches ask more of him and he cannot handle the broader role. In any event, the simple observation is pretty relevant and any QBs coach has to know these things about his player/s.
* 2004 would have only skewed it worse than it already is.