Definition of an Outlier

An observation that is unusually small or large relative to the other values in the data set is called an outlier.

Outliers are typically the result of one of the following causes:
  1. The measurement is observed, recorded, or entered into the computer incorrectly.
  2. The measurement comes from a different population.
  3. The measurement is correct, but represents a rare event.

-- ErinEsp - 15 May 2010