  The inherent connection between noise and disturbance is one of the most
fundamental features of quantum measurements. In the two well-known extreme
cases a measurement either makes no disturbance but then has to be totally
noisy or is as accurate as possible but then has to disturb so much that all
subsequent measurements become redundant.Most of the measurements are, however,
something between these two extremes.We derive a structural relation between
observables and channels that properly explains the trade-off between noise and
disturbance.
