Spearman's Rank Correlation
Spearman's rank correlation coefficient is used to measure the linear relationship between two continuous variables.
It takes values between -1 and 1. A positive coefficient means a positive linear relationship (i.e. both variables move in the same direction) while a negative coefficient means a negative linear relationship (i.e. the two variables move in the opposite directions). Both -1 and 1 refer to perfect linear relationship while 0 refers to no linear relationship.
Unlike the Pearson correlation coefficient, the Spearman's rank correlation coefficient does not assume the two continuous variables are distributed as Normal.