**Correlation** is a statistical measure that expresses the extent to which two variables are linearly related. It quantifies the degree to which a change in one variable is associated with a change in another variable. The correlation coefficient, often denoted as \( r \), can range from -1 to +1, where:
- \( r = 1 \): Perfect positive correlation, meaning that as one variable increases, the other variable also increases.
- \( r = -1 \):