A statistical number that represents the linear relationship between two variables or groups of variables (X and Y). The correlation has a value between −1 and +1. A positive value represents a positive linear relationship, whereas a negative value represents a negative relationship. A value of 0 represents no relationship between the variables. It is calculated from:
where n is the number of values. The correlation coefficient requires that the relationship between the two variables is linear. Where this is known not to be the case, then the correlation coefficient is not useful.