When approaching any form of data analysis many types and sources of error may be considered: the data collection procedure may contain errors; there may be gross data capture or encoding errors; there may be errors in the approach adopted to selecting data, designs or in analysis. However, none of these relates to the rather special use of the term statistical error. This term refers specifically to non-systematic or random errors that are observed during measurement. There may be many reasons why such random variations occur, but in general the assumption is made that these reasons are unknowable and therefore cannot be readily removed. Systematic variations and gross errors can, at least in theory, be separated out and either accounted for or removed, leaving simply statistical error.