there's this thing called variance. it happens all the time and is always there, especially whenever the outcome isn't zero sum.
its basically what prevents people from being able to predict the future based on new information and previous results in sporting contest and what not. we can make our best educated guess based on these factors but there are just two many outside variables that play a factor in the end sum for the result to ever come up identical every single time and be predicted with 100% accuracy.
in this case Chris Johnson ran for ~1200 yards, ~1300 yards, ~2,000 yards and lastly ~1,000 yards over a 4 year span. none of these individually is his true expectation and are merely fluctuations in variance based on X number of variables. when evened out they form what is called his expected rushing value, which (so far) is ~1400 yards.
now just because he had one severely below expectation year (the 1,000 yard year) doesn't mean that its a foreshadow of things to come. its actually the complete opposite as he just came off his highest above expectation total as a rusher and he should naturally regress back towards his mean.
basically, as long as no major variables have been changed within the test subject (chris johnson) he is still a heavy favorite to perform at or around his expected rushing value. in his case hes still in his athletic prime (26 years old), hasn't had any major physical injuries and still has a relatively decent o-line.
as for predicting his future expectation based on a loss of work ethic or whatever, one year is simply too small of a sample to guestimate this at any legitimate rate and basing your grade of him as a player based on this is simply flawed logic.
