Effect Size

The Pearson product-moment correlation coefficient is measured on a standard scale -- it can only range between -1.0 and +1.0. As such, we can interpret the correlation coefficient as representing an effect size. It tells us the strength of the relationship between the two variables.

In psychological research, we use Cohen's (1988) conventions to interpret effect size. A correlation coefficient of .10 is thought to represent a weak or small association; a correlation coefficient of .30 is considered a moderate correlation; and a correlation coefficient of .50 or larger is thought to represent a strong or large correlation.

You may have noticed that we have not discussed statistical tests of correlation coefficients. While we can conduct statistical tests on correlation coefficients, they are descriptive statistics that indicate the strength of relationship. The statistical test tells us whether the correlation is significantly different from zero; the absolute value of the correlation coefficient is an effect size that summarizes the strength of the relationship.

Effect Size (Cohen)



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