STA 2023 Final Exam Review Correlation Correlation is a mathematical relationship between two variables. A scatter plot is used to visually check for a linear relationship between two variables. The lower-case letter r is used as the symbol for the sample correlation coefficient. I measures the strength and direction of the linear relationship between two quantitative var Degrees of freedom are df = n - 2 The value of r is always between -1 and 1 inclusive. The closer r is to zero, the weaker the linear relationship. A positive correlation indicates that as one variable increases the other increases. A negative correlation indicates that as one variable increases the other decreases. Correlation is a mathematical relationship and does not imply causation of variables Things that adversely affect the value of r are extraneous variables, small sample, and measurement error, homogeneous grouping