# Correlation **Correlation** measures the strength and direction of a linear relationship between two variables. ## Pearson Correlation Coefficient $r=\frac{\sum(x_i-\bar{x})(y_i-\bar{y})}{\sqrt{\sum(x_i-\bar{x})^2 \sum(y_i-\bar{y})^2}}$ ## Spearman’s Rank Correlation