[S]uch comparisons tell an incomplete story. By comparing Clemens only to those who were successful in the second act of their careers, rather than to all pitchers who had a similarly successful first act, the report artificially minimizes the chances that Clemens will look unusual.
There’s a pretty neat trick at work here: if you compare Clemens only to those who had a terrific last decade of their careers, then the last decade of Clemens’ career doesn’t look that unusual. To sidestep this, we suggest that “[a] better approach to this problem involves comparing the career trajectories of all highly durable starting pitchers.”
So we put together data on all 31 other pitchers since 1968 who started at least 10 games in at least 15 seasons and have pitched at least 3,000 innings. This broader comparison group yields some pretty different conclusions than the Clemens v. Ryan contrasts.
The regression lines show the typical course of performance of aging pitchers; this shows exactly the reverse "u" curve of an athlete's performance as he ages. Clemens's career deviates from the trajectory significantly.
This statistical deviation does not prove Clemens ever used PEDs. It might mean Clemens is a once-in-a-lifetime athlete who can maintain incredible performance as he ages. Or it might mean he used a performance-enhancing agent -- that doesn't mean a drug.
The bottom line: looking at the curves generates a question of how Clemens defies performance-aging gravity.