Another measure of the quality of fit is the correlation coefficient, r². To calculate the correlation coefficient, square the total of the (x-)(y-) column, and divide by the total of the (x-)² and the total of the (y-)² column. The formula is:

r² equals 1 if the data all lie exactly along a straight line, and r² equals 0 if the data are not correlated. Values between 1 and 0 indicate that the data have some linear relationship, but also have some scatter. Data with an r² of above .8 are considered strongly correlated.