  We study macroscopic observables defined as the total value of a physical
quantity over a collection of quantum systems. We show that previous results
obtained for infinite ensemble of identically prepared systems lead to
incorrect conclusions for finite ensembles. In particular, exact measurement of
a macroscopic observable significantly disturbs the state of any finite
ensemble. However, we show how this disturbance can be made arbitrarily small
when the measurement are of finite accuracy. We demonstrate a tradeoff between
state disturbance and measurement coarseness as a function of the size of the
ensemble. Using this tradeoff, we show that the histories generated by any
sequence of finite accuracy macroscopic measurements always generate a
consistent family in the absence of large scale entanglement, for sufficiently
large ensembles. Hence, macroscopic observables behave "classically" provided
that their accuracy is coarser than the quantum correlation length-scale of the
system. The role of these observable is also discussed in the context of NMR
quantum information processing and bulk ensemble quantum state tomography.
