[Note: As the Adrian Peterson saga drags on, now is a perfect time to continue the three-part study Brad Davis has put together on the likelihood of decline from Adrian Peterson. Previously, we looked at Adrian Peterson’s statistical output alone when determining whether or not he’s declining, but in Part Three, Brad Davis looked at 33 different running backs to see the commonalities in running back decline and how they apply to Adrian. Davis, who doesn’t take a stance on Peterson’s trade value in these pieces, ran through the data with rigorous statistical analysis. Part One of the study, which looks solely at Peterson’s career, is here. Part Two of the study, which answers criticisms and questions of part one, is here.]
by Brad Davis
Everybody who follows football beyond the casual observer ‘knows’ that running backs, ALL running backs, performance declines with age. And everybody makes reference to the fact that this decline begins around age 30. This fact was brought up again and again in the discussion of whether Adrian Peterson has shown significant decline in his performance or whether we should expect to see this decline shortly.
In the first two blog posts discussing whether Adrian Peterson has shown any evidence of decline in his play over the course of his career, we evaluated his performance using four different metrics: Yards per attempt, WPA, and two different success rate measures: DD (down and distance), and AFA over two different time scales: his entire career, and the three most recent years in his career. And we didn’t see any evidence of decline in his performance for any of those four metrics over either time scale.
However in the course of that analysis and over the course of some discussion on forums and twitter, we began to wonder if it would be possible to detect any evidence for the decline in the performance of any running back using these metrics? After all, if everyone knows that all running backs decline and our metric for measuring decline is any good, we should be able to detect it, right?
With that in mind, we set out to see if the data supports this widely held view, not using conjecture, not using our eyes, but using rigorous statistical analysis of running back performance data. This was a pretty major undertaking. We obtained running back performance data for 33 running backs who have more than 8 years of history playing in the NFL who started playing in 1998 or later. This list includes: Adrian Peterson, Ahmad Bradshaw, Ahman Green, Brandon Jacobs, Brian Westbrook, Cedric Benson, Chester Taylor, Clinton Portis, DeAngelo Williams, Deuce McAllister, Dominic Rhodes, Edgerrin James, Frank Gore, Fred Jackson, Fred Taylor, LaDainian Tomlinson, LaMont Jordan, Larry Johnson, Marshawn Lynch, Maurice Jones-Drew, Michael Turner, Mike Anderson, Pierre Thomas, Reggie Bush, Ricky Williams, Ron Dayne, Ronnie Brown, Rudi Johnson, Sammy Morris, Shaun Alexander, Steven Jackson, Thomas Jones, and Willis McGahee.