When do you take a RB?

At the WSJ (via Right Field), there's a short article about when to take a running back in the NFL draft. An excerpt:

In recent years, drafting a running back in the first round hasn't been a worthwhile investment at all. Since 2002, 28 backs have been selected in the first round. Less than a third have ever made a Pro Bowl; on average, they gain a so-so 730 yards per season.

There's more data at the link. I looked into it a little more myself and using other measures the data holds up. For example, here are the leading rushers in the NFL last season. Of the top ten only four (7-10) were taken in the first round. Only two of those were picked earlier than #20 (Marshawn Lynch and Ryan Mathews). The sweet spot for taking a running back? Early second round.