Statistics – The Normal Distribution

A large part of Statistical Reasoning in the Medical Sciences depends on the special characteristics of the Normal Distribution. 


This Bell Shaped curve is the archetypical illustration of the Normal distribution.

[ This image is from COSMIC IMAGERY – Key images in the history of Science.  John D Barrow.  2008. The Bodley Head. London.  ISBN 978-0-224-07523-7 ]

The Normal distribution (capitalised to show it is now accepted as a proper Noun) is the most important numerical distribution in relation to biological phenomena.

It features frequently in medical statistics.  An understanding of the characteristics of the Normal distribution is crucial to Dental Age Estimation.

The definition is ” A probability distribution assumed by many statistical procedures. The distribution is bell-shaped and depends on two parameters, the population mean and the population variance. ”  [ Everitt B.  2006. A Guide to Medical Statistics  from A to Z.  A guide for Clinicians and medical students.  2nd Edition. Cambridge University Press. Cambridge. ISBN 0-521-687-68718-7 ]

The graphabove clearly demonstrates the bell-shaped appearance of a classic probability density histogram.

To illustrate the important aspects of the Normal Distribution the data for the UK-Caucasian Reference Data Set will be used viz. LL8Gf [ The Lower Left Third Molar, Stage G female ]