A standardized measure of the linear association between two variables. Its values range from –1, indicating a strong negative relationship, through zero, which shows no relationship, to +1, indicating a strong positive association. The correlation coefficient is the covariance between a pair of standardized variables. Curtis and Alf (1969) and Ozer (1985) argue that *r* is a better measure of predictive ability than *R ^{2}* (but neither is very useful for time-series data). A strong correlation does not imply a causal relationship.

- Curtis, E. W. & E. F. Alf (1969), “Validity,
predictive efficiency, and practical significance of selection tests,”
*Journal of Applied Psychology*, 53, 327-337. - Ozer, D. J. (1985), “Correlation and the coefficient of
determination,”
*Psychological Bulletin*, 97, 307-315.