A Matrix Formulation on How Deviant an Observation Can Be

There have been many studies on the detection of outliers.In one of the very early articles it was shown that the deviation of any particular observation from the mean is bounded by a multiple of the standard deviation.This result is simple and has resulted in periodic rediscoveries, coupled with a variety of alternative proofs.We provide a historical review of these various methods, and then pose yet another proof that may have some intrinsic interest.