Pearson Correlation Coefficient

The Pearson correlation coefficient, also called Pearson's r or the Pearson product-moment correlation coefficient, is a correlation coefficient that measures linear correlation between two sets of data. It is the ratio between the covariance of two variables and the product of their standard deviations, so the result is a normalized value that always falls between minus one and one and carries no units, which lets the strength of association be compared across different pairs of variables. Karl Pearson developed the coefficient from ideas introduced by Francis Galton in the 1880s, though the underlying mathematical formula was first derived and published by Auguste Bravais in 1844, an example of what is known as Stigler's Law of eponymy.