Normalized deviation কী?

Two results $x_1 \pm u_1$ and $x_2 \pm u_2$ of the same measurand are compared through their normalized deviation

They are compatible when $z \leq 2$; a larger $z$ points to an unaccounted effect — or an underestimated uncertainty. When $x_2$ is a reference value with no quoted uncertainty, take $u_2 = 0$.