  Employing different statistical ensembles may lead to qualitatively different
results concerning averages of physical observables on the mesoscopic scale.
Here we discuss differences between the canonical and the grandcanonical
ensembles due to both quenched disorder and thermodynamical effects. We show
how these differences are related to spectral correlations of the system at
hand, and evaluate the conditions (temperature, system's size) when the
thermodynamic limit is achieved. We demonstrate our approach by evaluating the
heat capacity, persistent currents and the occupation probability of single
electron states, employing a systematic diagrammatic approach.
