A number, mathematically represented by "r," that is a quantification of the linear relationship that exists between two sets of paired data or two random variables. It is equal to their covariance divided by the product of their standard deviations and has a two-decimal value ranging between -1.00 and +1.00. The nearer the number is to 1.00 (plus or minus), the stronger is the correlation (either positive or negative). See text, Chapter 13. See also, "correlated," "mean," "predictive reliability," "regression" and "standard deviation."