Abstract: A method and apparatus for determining on-machine, components of variances over an entire frequency range, of a property of a substantially continuous moving sheet material. At least two property-sensing detectors measure and each provide a signal Y(t) proportional to a sheet material property at a different location in the cross machine direction of a moving sheet material. Preferably a spectrum analyzer, determines the power spectral density function G(f) for each detector's signal, and the coherence function COH(f) between the two detectors' signals. On the basis of these results, the total variance, the machine direction variance, the residual variance and the contributions of the machine-directional cyclic and non-cyclic variances of the property of interest are determined, preferably by a computer. These contributions are generally indicative of process and/or machine upsets.