If anything is characteristic of biological signals it is that they fluctuate. Yet, like the staggering progress of science, there is direction and correlation. We often need to characterize correlations, that is, the degree and extent of similarity of a measured property as it varies in space and time. The use of the normal curve assumes the fluctuations are independent, not correlated. However, in nature these fluctuations are more structured. For example, structured correlations occur in the variation in density of water in a cloud, the variation in regional blood flow, and local receptor densities in an organ.
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