In statistics, the Pearson product-moment correlation coefficient (sometimes referred to as the PPMCC or PCCs, and typically denoted by r) is a measure of the correlation (linear dependence) between two variables X and Y, giving a value between +1 and −1 inclusive. It is widely used in the sciences as a measure of the strength of linear dependence between two variables. It was developed by Karl Pearson from a similar but slightly different idea introduced by Francis Galton in the 1880s. The correlation coefficient is sometimes called "Pearson's r."
Pearson's is the sum of each observation minus its mean squared over the product of the standard deviation of each variable.