This measures the level of linear relationship between two variables. It lies in the range [-1,+1] where +1 denotes a perfect positive relationship and -1 a perfect negative relationship. It is calculated by working out the covariance (average product) of the two variables and dividing by the product of their individual standard deviation (s.d., σ): ( cov(x,y) / ( σ_{x} × σ_{y}).

Used on pages 123, 124

Also known as correlation coefficient