A number, written k, that multiplies the combined standard uncertainty to produce the expanded uncertainty. For a normal distribution k = 2 gives about 95.45 % coverage, k = 1.96 gives exactly 95 % and k = 3 gives about 99.7 %. When the effective degrees of freedom are small, k comes from the Student t-distribution instead, for example k = 2.28 for 95.45 % coverage with 10 degrees of freedom.
Effective degrees of freedom came out at 9, so the coverage factor is 2.32, not 2, fix the cert template.
Also heard as
- k
- k-factor
Do not mix it up with
- Sensitivity coefficient: Sensitivity coefficients scale individual inputs inside the budget; the coverage factor scales the final combined result.
Related terms
Expanded uncertainty
Combined standard uncertainty multiplied by a coverage factor to give an interval with a stated coverage probability.
Student t-distribution
A bell-shaped distribution with heavier tails than the normal, used to set coverage factors when data are limited.
Welch-Satterthwaite formula
The equation that estimates effective degrees of freedom for a combined uncertainty, used to pick the coverage factor.
Coverage probability
The probability that the coverage interval contains the measurand's value, usually stated as 95 percent.
Two sigma
Shop shorthand for an uncertainty quoted at k = 2, roughly 95 percent coverage.