The correlation coefficient, also called Pearson's correlationcoefficient and labelled
teststhe goodness of fit of a set of points to a straight line.
isbetween -1 and 1.If
thepoints are a perfect fit to a straight line with a positivecorrelation and if
theyare a perfect fit to a line with a negative correlation.
Spearman's rank correlation coefficient
onlytests if tow quantities increase or decrease together, withoutnecessarily being a good fit to a straight line. A value of 1formeansthat if one quantity increases then so does the other with absolutecertainty.
Spearman's rank correlation coefficient is preferable is twoquantities are related but the relationship is not linear. It isusual to draw a graph before assessing the nature of anyrelationship.