E.1 Distribution functions
The following table shows how to calculate a standard uncertainty from the parameters of the two most
important distribution functions, and gives an indication of the circumstances in which each should be used.
EXAMPLE
A chemist estimates a contributory factor as not less than 7 or more than 10, but feels that the value could be
anywhere in between, with no idea of whether any part of the range is more likely than another. This is a
description of a rectangular distribution function with a range 2a=3 (semi range of a=1.5). Using the function
below for a rectangular distribution, an estimate of the standard uncertainty can be calculated. Using the above
range, a=1.5, results in a standard uncertainty of (1.5/√3) = 0.87.