Median
The middle value of a replicate set of results.
Degree of Freedom
An independent variable. The number of degrees of freedom possessed by a replicate set of results
equals the total number of results in the set. When another quantity such as the mean is derived from the
set, the degrees of freedom are reduced by one, and by one again for each subsequent derivation made.
Precision
The variability of a measurement. As in the case of error, above, it may be expressed as an absolute or
relative quantity. Standard deviations are the most valuable precision indicators (vide infra).
Spread
The numerical difference between the highest and lowest results in a set. It is a measure of precision.
Deviation (e.g. From the Mean or Median)
The numerical difference, with respect to sign, between an individual result and the mean or median of
the set. It is expressed as a relative or absolute value.
Standard Deviation, σσ
A valuable parameter derived from the normal error curve (p. 16) and expressed by:
where xi is a measured result, μ is the true mean and N is the number of results in the set. Unfortunately,
μ is never known and the mean derived from the set of results has to be used. In these circumstances
the degrees of freedom are reduced by one and an estimate of the true standard deviation is calculated
from: