You can turn on your television and see this year's NFL prospects hopping, skipping and jumping all around Lucas Oil Field in Indianapolis. You can look online and find tables full of numbers, from heights and weights to shuttle times and bench press totals. But what does any of it really mean?
At Football Outsiders, we're not really interested in a bunch of numbers if they don't mean anything. So we did some research to figure out which combine performances really matter. No combine figure is ever going to be a foolproof indicator, but we have found that some drills are worth paying attention to at certain positions. For running backs, that drill is the 40-yard dash. Bill Barnwell, the former FO writer who now writes for Grantland, took the 40, adjusted it based on player weight and came up with a metric that has a surprising level of relevance: Speed Score.
On its own, 40 time correlates with future success less than some fans might expect. But not all 40 times are created equal because not every player is running with the same body. When a 225-pound player runs a 4.48 40-yard dash, it's a lot more impressive than that same 40 time from player who weighs 185 pounds. On top of that, the range of 40 times for running backs is so small (from about 4.2 seconds to 4.9 seconds) that even a minuscule difference can be valuable. Times of 4.41 and 4.51 might look roughly similar, but, in the NFL, holes can close up just that quickly.
Adjust for those factors and you get the formula for Speed Score: (weight * 200)/40 time^4. Multiplying the player's weight by 200 conveniently scales the metric so that an average Speed Score is right around 100. The average first-round pick approaches a Speed Score of 112.
Since 1999, Speed Score has been able to explain about 20 percent of the variance between NFL running backs in standard stats such as rushing yardage and in advanced stats such as Football Outsiders' DYAR (defense-adjusted yards above replacement).
In the past, Speed Score has pointed to the future success of middle-round picks such as Brandon Jacobs (123.5) and undrafted free agents such as LeGarrette Blount (105.8). It has pushed speedy backs such as Chris Johnson (121.9) to the top of their classes and suggested disappointing careers for future busts such as William Green (98.7) and Trung Canidate (99.3). Last year, it suggested that Roy Helu (114.8) and DeMarco Murray (112.6) were middle-round sleepers.