Advice: Rank Correlation
Interval (non-normal) vs. Interval (non-normal)
KENDALL'S RANK CORRELATION: Kendall's and Spearman's methods for rank correlation listed in the nonparametric methods section of the analysis menu. You are advised to use Kendall's rank correlation as this reflects the strength of the relationship between the variables and copes with ties much better than Spearman's method. As a test of independence Kendall's method is also superior because it is sensitive to some types of independence which can not be detected using Spearman's method. Do not use Spearman's method at all if either variable is anything less than ordinal.