One of the difficulties in trying to grade players by statistics is simply gauging the impact that certain factors have on how those numbers were tallied.
For example, one of Bill James' early sabermetric findings revolved around detailing just how much a stadium could influence a player's statistics; he pointed out the effects that hitter-friendly Fenway Park had on Jim Rice and Fred Lynn before most everyone else had ever considered that idea.
There are certain ways that type of thinking can be applied to football. In the case of running backs, one of the methods I use is the good blocking yards per attempt (GBYPA) metric.
The specifics of this system are quite detailed -- but the nutshell explanation is that I grade tape of every block on every running play. Each block is given a win or loss grade. A win grade roughly means that the blocker moved the defender out of the point of attack. A loss grade generally indicates that the defender beat the blocker.
I have used this system for a couple of seasons and have found that no running back gains more than three yards per attempt on plays where there is at least one POA blocking loss. That means the key to running back success is to gain as many yards as possible on plays with good blocking (i.e. plays where all of the blockers win their POA battles).
When the rushing totals are parsed this way, they often yield some interesting results. Last season was no different, as there are some big surprises both in the ballcarriers who did well in the GBYPA metric and those who did poorly (information that can be quite helpful if you have a fantasy football draft coming up this weekend).