  Incompatible observables can be approximated by compatible observables in
joint measurement or measured sequentially, with constrained accuracy as
implied by Heisenberg's original formulation of the uncertainty principle.
Recently, Busch, Lahti, and Werner proposed inaccuracy trade-off relations
based on statistical distances between probability distributions of measurement
outcomes [Phys. Rev. Lett. 111, 160405 (2013); Phys. Rev. A 89, 012129 (2014)].
Here we reform their theoretical framework, derive an improved relation for
qubit measurement, and perform an experimental test on a spin system. The
relation reveals that the worst-case inaccuracy is tightly bounded from below
by the incompatibility of target observables, and is verified by the experiment
employing joint measurement in which two compatible but typically
non-commutative observables on one qubit are measured simultaneously.
