 Expanded uncertainty

An expanded uncertainty is $U=k{u}_c$, where $u_c$ is the combined standard uncertainty and $k$ a stated coverage factor. A result written $y\pm U$ aims to describe an interval with a specified coverage interpretation.

If a normal approximation is appropriate and the standard uncertainty is well determined, $k\approx 2$ gives approximately 95 percent coverage. This interpretation requires those assumptions; multiplying by two alone does not prove a coverage probability. Report the factor and the basis for its choice.^[ref]