What's good on the goal line is good on the gridiron

Grantland's Bill Barnwell continues his examination of NFL running backs and again, he wants to talk about the Titans, specifically: why in the heck did they sign Shonn Greene?

The prima facie case for Greene is that he can handle short-yardage situations — the Smash to Chris Johnson's Dash — alleviating some of the pressure and wear and tear on No. 28.

What Barnwell found, though, is that Greene converts goal-line carries into touchdowns at a rate lower  than the NFL average:

On first down with one yard (or less, in this case) to go for a touchdown, Greene has received seven carries. Since 51 percent of carries on first-and-goal from the 1-yard line by a running back from 1999 to 2012 produced a touchdown, our expectation is that Greene would have scored 3.6 touchdowns, since (7 * .51 = 3.57). However, in reality, Greene has only scored one touchdown, so on that down and distance, he has scored 2.6 touchdowns below an "average back" given those same carries. When you add up all those opportunities in each of the situations in question, Greene's actually a below-average goal-line back, having scored 11 touchdowns against an expectation of 12.2 scores.

Than Barnwell tries to discover what makes a "good" goal-line back. He checks for correlation between height, weight and BMI and finds them all wanting. So what makes a good goal-line back?

History doesn't tell us that there are any particular characteristics for a great goal-line back beyond, well, being a great running back in other situations and having a very good offensive line to work with. That doesn't seem like it tells us much, but it might have been enough to convince the Titans that they didn't need to give Shonn Greene all that money to fill a job he hasn't been particularly good at.