This graphic, originally published on June 16, 2013, allows you to compare performance ratings for different quarterbacks at different ages.To create the performance ratings, the Globe used a formula developed by David Berri, a sports economist and professor of economics at Southern Utah University. The formula (Total Yards) - 3*(Passing Attempts) - 30*(INTs) is based on statistics compiled for every season a quarterback played. Quarterbacks eligible for the statistical sample started at least 10 games in at least 8 seasons from 1960 to present. The equation aims to measure player performance by linking statistics to wins and to mitigate factors such as team strategy so QBs can be equitably compared.
Yale professor Ray Fair used the performance ratings to estimate the effects of aging on NFL quarterbacks. Fair emphasized that many other variables affect how an athlete performs in a given year aside from his age.