An ensemble is a collection of systems, all prepared in the same way. Does this mean that all the systems are in the same state? I have seen some authors create ensembles where 30% of the systems are in a state, s, and 70% of the systems are in a state, t . As far as measurements go, this ensemble represents the mixed state 0.3 s + 0.7 t . Is this accepted as a valid ensemble for this mixed state?