SECTION 2.2 Summary

• The correlation measures the strength and direction of the linear association between two quantitative variables and . Although you can calculate a correlation for any scatterplot, measures only straight-line relationships.
• Correlation indicates the direction of a linear relationship by its sign: for a positive association and for a negative association.
• Correlation always satisfies and indicates the strength of a relationship by how close it is to or 1. Perfect correlation, , occurs only when the points on a scatterplot lie exactly on a straight line.
• Correlation ignores the distinction between explanatory and response variables. The value of 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 .