Correlation
Lead Author(s): NancyHills
Measures of correlation are not statistical tests of inference, but are actually descriptive statistical measures which represent the degree to which two or more variables are related to one another. After calculating a measure of correlation, such as the Pearson product-moment correlation coefficient or the Spearman’s rank correlation, an inferential statistical test is often used to evaluate hypotheses regarding the correlation coefficient. E.g., we may wish to test the null hypothesis that a correlation between two variables equals 0.
Correlation is concerned with trends: if X increases, does Y tend to increase or decrease? How much? How strong is this tendency?
doc 
PNG 