#moment.statistic  the n-th moment of a distribution is the expected value of the n-th power of the deviations from a fixed value
  supertype:  statistic  a datum that can be represented numerically
  subtype:  second_moment  the expected value of the square of the deviations of a random variable from the point of origin
  subtype:  variance  the second moment around the mean; the expected value of the square of the deviations of a random variable from its mean value
     subtype:  standard_deviation  the square root of the variance
     subtype:  covariance  statistical measure of the variance of two random variables measured in the same mean time period

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