While a good 40-yard dash time is potentially worth millions and millions of dollars to potential NFL draft picks, there's good reason to doubt the scientific validity of the entire process.
In a fascinating 2005 article from the San Diego Union-Tribune, h/t @MattRHinton, Mark Ziegler talks to a bunch of people within the world of track, who point out that the way NFL scouts time football players is pretty suspect, from using their thumb (which has a slower reaction time) instead of their index finger to not having a starter's gun, automatic timing device or a wind gauge.
But, as Ziegler points out, the real red flag is the fastest recorded 40 in track history, the first 40 yards of Ben Johnson's gold-medal winning 100 meter dash in the 1988 Olympics:
He was running in spikes . . . on a warm afternoon perfectly suited for sprinting . . . with a slight tailwind . . . with years of training from arguably track's top coach, Charlie Francis . . . with Carl Lewis and six others of the fastest men on the planet chasing him . . . with 69,000 people roaring at Seoul's Olympic Stadium . . . with hundreds of millions of people watching on TV . . . with the ultimate prize in sports, an Olympic gold medal, at stake.
And, as we learned later, with muscles built with the assistance of the anabolic steroid stanazolol.
Then again, maybe Ben Johnson isn't the fastest 40-yard man in the world.
Maybe half the NFL is faster.
While a lot of these errors may even out when it comes to comparing 40 times between NFL prospects, it's something worth keeping in mind as Combine workouts dominate draft coverage.