• The correlation r measures the direction and strength of the linear (straight line) association between two quantitative variables x and y. Although you can calculate a correlation for any scatterplot, r measures only linear relationships.
• Correlation indicates the direction of a linear relationship by its sign: r > 0 for a positive association and r < 0 for a negative association.
• Correlation always satisfies −1 ≤ r ≤ 1 and indicates the strength of a relationship by how close it is to −1 or 1. Perfect correlation, r = ±1, occurs only when the points lie exactly on a straight line.
• Correlation ignores the distinction between explanatory and response variables. The value of r is not affected by changes in the unit of measurement of either variable. Correlation is not resistant, so outliers can greatly change the value of r.