A measure of dispersion; for n readings the experimental standard deviation s is the square root of the sum of squared deviations from the mean divided by n minus 1. In uncertainty work it describes the spread of individual readings, which is repeatability, and it is the form every standard uncertainty takes. The standard deviation of the mean, s divided by the square root of n, applies when the reported value is the average of those readings.
Standard deviation of the ten readings is 0.6 grams, but we report the mean, so that's 0.19 in the budget.
Divide by root n only when the result really is an average of those n readings; a customer who will take one reading needs the single-reading scatter.
Also heard as
- SD
- s
- sigma
- experimental standard deviation
Related terms
Type A evaluation
Putting a number on an uncertainty component by running statistics on your own repeat readings.
Standard uncertainty
Uncertainty expressed as one standard deviation, the common currency for combining uncertainty components.
Repeatability
How tightly readings bunch when one person repeats a measurement with the same setup, in one place, in one short sitting.
Normal distribution
The bell-shaped distribution assumed for most certificate values and averaged readings, set by mean and standard deviation.
Degrees of freedom
A count of the independent information behind an uncertainty estimate, showing how reliable that estimate is.