I did a Wilcoxon rank sum test to figure out if there is any difference in the distribution of Variable $X_1$ for two groups. The test results showed that the difference does exist.
I, however, suspect that there is a confounding variable $X_2$, which is causing this difference.
How can I control for $X_2$?
Joe wants to understand if there is any difference in the listening capabilities of Males and Females. Joe thinks that "Age" could be a confounding variable in his study; so he wants to control for "Age". Is there any non parametric test that Joe can conduct?