When I first started writing at my old blog I was always amazed back in 2003 and 2004 that QB Brett Favre was still among the league leaders in TD passes despite his advancing age. Arguably his 2004 and 2007 seasons were the best of his career, although I'll still stick with his MVP seasons (1995-1997) as his best. Numbers guru Aaron Schatz takes a look back at Favre's career and explains that although his peak seasons were not among the greatest of all time, his consistency and longevity are one of a kind. The entire article is great, but the following is the key part:
At Football Outsiders, we use our complex DVOA (Defense-adjusted Value Over Average) metric to break down every single play of the season and figure out how good each player is compared to the average player at his position. Our play-by-play database goes back to 1995, so it includes most of Favre's career. Only once, in 1999, does Favre come out as a below-average quarterback -- and just barely.
In 2005, Favre threw a league-leading 29 interceptions, and most observers believed he had hit the end of the line. Yet even that year, Favre was an above-average quarterback when you consider his team's schedule. Green Bay played six games that season against the four defenses with the most interceptions -- Cincinnati, Chicago, Minnesota and Carolina -- and another two games against Detroit, which ranked seventh.