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 R² (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.