Many approaches to estimate the correlation of data have been developed. One of the most commonly used correlation measures is the Pearson (linear) correlation coefficient -which quantifies the strength of the relationship between two variables as a value between -1 and 1. A Pearson correlation value of 1 indicates perfect correlation, a value of 0 indicates independence of the variables, and a value of -1 indicates perfect anticorrelation. The Pearson product-moment correlation is defined as