Can I use Pearson correlation for discrete variables? Can I use Pearson correlation for discrete variables, if the level of measurement fits (either ratio or interval).
Thanks!

It depends on what you mean in "discrete". You can use Pearson's R for discrete numeric variable. If the data are in ordinal scale, you should use Spearman's correlation. If the data are in nominal scale, then Pearson's and Spearman's coefficients are not valid, even if you code the categories by numbers.
