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Results of statistical correlation measurements for obtaining information in a complex system are presented. Experiments were performed on four nuclear power plants using conventional plant instrumentation. Inherent fluctuations (noise), response to forced random, sinusoidal and step variations in reactivity were studied. Frequency domain analysis of the data was performed with a special purpose analog type analyzer for determining power and cross-power spectral densities. Results obtained demonstrate that 1) significant semi-quantitative information on the process can be obtained from power spectral analysis on inherent fluctuations: and 2) cross-power spectral techniques permit frequency response measurements and system parameter determination in commercial nuclear plants.