A measure of the strength and direction of the linear relationship
between two variables
that is defined as the (sample) covariance of the variables
divided by the product of their (sample) standard deviations.
n∑⟮xy⟯ − ∑⟮x⟯∑⟮y⟯
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√［（n∑x² − ⟮∑x⟯²）（n∑y² − ⟮∑y⟯²）］