  The concept of experimental accuracy is investigated in the context of the
unbiased joint measurement processes defined by Arthurs and Kelly. A
distinction is made between the errors of retrodiction and prediction. Four
error-disturbance relationships are derived, analogous to the single
error-disturbance relationship derived by Braginsky and Khalili in the context
of single measurements of position only. A retrodictive and a predictive
error-error relationship are also derived. The connection between these
relationships and the extended Uncertainty Principle of Arthurs and Kelly is
discussed. The similarities and differences between the quantum mechanical and
classical concepts of experimental accuracy are explored. It is argued that
these relationships provide grounds for questioning Uffink's conclusion, that
the concept of a simultaneous measurement of non-commuting observables is not
fruitful.
