So I went to the data to try and determine if there was a trend in the data to support this.
The metric I chose to look at is “yards gained per attempt,” or YG/A, by game, versus the season average. YG/A seems like a nice measure because it is affected by both a QB’s accuracy as well as their completion distance. Throw long bombs YG/A goes up, throw a bunch of short or incomplete passes and YG/A goes down.
The process to look at the data is simple: look at an entire season and find the average Yards Gained per Attempt over the entire season. Then plot each game and draw a trend line to see if it goes down over the course of the season.
Lo and behold, starting in 2009 (age 33) through 2013 the slope of the (Black) line goes negative whereas it had been almost completely positive from (2002 – 2008). So maybe there is something to a QB getting older and playing worse as the season goes on. However in 2014 the line is nearly flat because of a single big game at the end of the season.
Obviously this doesn’t control for strength of opponents by week, but interesting given that Peyton sat for 2011 and the slope of the line is negative for 2012, and 2013 i.e. Resting for a year didn’t change the trend of declining performance over the season.
The plots show YG/A plots for the first 14 games of each of Peyton’s regular seasons. (I got rid of the last two games each season in effort to ignore any odd situations where having locked up a playoff spot might exert a strong influence on performance) Each plot shows YG/A by game, minus the season avg. An above zero game e.g. 2.1 would indicate that in that game, Peyton’s Yards Gained Per Attempt was 2.1 yards better than the season average. A trend line with a downward slope would indicate deteriorating performance over the season. i.e. yards per attempt dipping below the season average towards the end of the season.