How buttery are your back's fingers?

Running backs are often ranked by their yardage, touchdowns or yards per carry, but those numbers should be achieved without putting the ball at risk. After all, remember that famous Monday Night "The Bears are who we thought they were" at Cardinals game? Devin Hester returning a punt for the win is the memorable play, but it was set up by a critical Edgerrin James fumble, also returned for a Bears' touchdown. What is the right metric for ball security, and how do you know if your back is doing well?

It is clearly not a good idea to look at total fumbles, and looking at carries per fumble isn't entirely telling either because it doesn't differentiate players with no fumbles and few carries (Garrett Wolfe's 15 in 2008) from those with proven performance over the season (LaDanian Tomlinson's 315 carries in 2007).

A better metric is the number of carries between fumbles, which I'll call the Mean (Average) Carries Between Fumbles:
MCBF = Carries / (Fumbles + 1)

Ranking the top 40 (by number of attempts) runners over the last 5 years shows us a few things:

MCBF of 70 is the median for a feature back

MCBF of 50 is fairly weak

Very few backs achieve MCBF over 150 in a season

Looking through the data bears out a few facts we already knew: LT's value is amplified by his stellar MCBF (315 and 292 the last two years), Brian Westbrook is consistenly reliable with an MCBF over 118 in each of the last 5 years, and Adrian Peterson is among the worst at holding onto the football (MCBF of 48 and 36 in his first two years). Perhaps there's more to the Vikings' inability to close games than suspect quarterback play? We also see that Shaun Alexander's problems were not limited to an increasingly tentative running style.

MCBF is also a strong indicator of a player's ability to contribute early. Marshawn Lynch and Matt Forte had eerily similar rookie seasons, both generally considered highly productive. In fact, neither had a tremendous average (less than 4.0 yards per carry), but both had stellar MCBF and could thus be counted on to move the chains and retain possession.