One point not explicitly made by Stuart but brought up by Brian Burke at Advanced Football Stats, is that because the NFL measures everything rounded to an yard, simple accuracy is missing in a lot of running back yardage stats. When you are trying to decipher whether a drop from a 4.5 ypc to 4.1 is a bad omen, it doesn't help that a lot of the fluctuation could be rounding.
It could be, but isn't it more likely that rounding would even itself out over the course of a season?
Ring the bells that still can ring
Forget your perfect offering
There is a crack, a crack in everything
That's how the light gets in - Leonard Cohen