  In recent years, novel quantifications of measurement error in quantum
mechanics have for the first time enabled precise formulations of Heisenberg's
famous but often challenged measurement uncertainty relation. These relations
take the form of a trade-off for the necessary errors in joint approximate
measurements of position and momentum and other incompatible pairs of
observables. Here we review some of these error measures, examine their
properties and suitability, and compare their relative strengths as criteria
for "good" approximations.
