Number of independent partial structure factors for a disordered n-component system

n(n+1)/2 independent diffraction measurements are needed to obtain complete partial structure information for a disordered n-component condensed system. A previous assertion that only n of these are independent is shown to be inapplicable, for different reasons, to both the three-dimensional partial structure factors Sab(Q) and the orientational averages Sab(Q). The fluctuations of Sab(Q) about Sab(Q) and the effects of thermal motion are discussed. copyright 1999 The American Physical Society