The Pearson correlation coefficient is the most widely used. It measures the strength of the linear relationship between normally distributed variables. When the variables are not normally distributed or the relationship between the variables is not linear, it may be more appropriate to use the Spearman rank correlation method. There is a very interesting paper about the differences between these two correlation coefficients on the same sets of data: http://geoinfo.amu.edu.pl/qg/archives/2011/QG302_087-093.pdf Hope it will be useful!