From AMS Glossary
Revision as of 17:54, 20 February 2012 by Perlwikibot
The magnitude of a deviation from a statistic that will be exceeded with probability of 0.50, or on half the occasions.
For a normal distribution it is 0.6745 times the standard deviation. The probable error is not "probable" in any peculiar sense and should have no more significance attached to it than the above.