Correlation coefficient
A number between −1 and 1 measuring the strength and direction of a linear relationship.
Pearson's correlation coefficient r is +1 for a perfect upward straight line, −1 for a perfect downward straight line and 0 for no linear relationship. Its square r² is the fraction of the variation in y explained by a linear fit in x.
Formula
r = Σ(x − x̄)(y − ȳ) / √(Σ(x − x̄)² · Σ(y − ȳ)²)