Lab-floor slang for an expanded uncertainty with a coverage factor of 2, borrowed from the normal distribution, where ±2 standard deviations covers about 95 % of values. Techs say a number is 'at two sigma' to flag that it is already expanded and must be halved before going into a budget as a standard uncertainty. It is loose usage, because sigma strictly means a population standard deviation and real coverage depends on degrees of freedom.
That 3 ppm on the cert is two sigma, so carry 1.5 into the budget.
Also heard as
- 2 sigma
- 2-sigma
Related terms
Coverage factor
The multiplier applied to combined standard uncertainty to obtain expanded uncertainty, usually k = 2.
Expanded uncertainty
Combined standard uncertainty multiplied by a coverage factor to give an interval with a stated coverage probability.
Normal distribution
The bell-shaped distribution assumed for most certificate values and averaged readings, set by mean and standard deviation.
Standard deviation
The usual measure of scatter in a set of values, the square root of the average squared deviation from the mean.