The descriptive statistics presented thus far involve the description of a single variable. Measures of relationship indicate the degree to which two variables are related. Correlation is often used for this purpose. The two most frequently used correlational statistics are the product-moment correlation coefficient (r) and rank-difference correlation (rho). For other bivariate correlational techniques (i.e., relationship between two variables), such as the phi coefficient, biserial correlation, correlation ratio (eta), partial correlation, and others,