Chris Shepherd's most recent PSA on drafting running backs spurred a lot of discussion and nuanced arguments. There is not a purely analytical way to identify which position to draft when, as varies by team need, relative draft strength of position, and the individual skill set of the player being considered.
However, using an analytical approach can help estimate a baseline value for drafting early/late for a specific position and thus provide a starting point to valuing a specific pick.
Using regular season data from 2000 - 2016 I plotted average rush yards per game for running backs by the round in which they were drafted.