A clinical trial is conducted on a specific drug used to treat severe migraines and it is noted that approximately
25% of individuals in the study fail to respond to the drug and the medication appears to give them severe episodes of nausea and vomiting. Upon further examination it is revealed that these individuals carry a genetic marker, and the individuals who did not experience the side effects of nausea and vomiting did not have the same marker as those individuals who experienced the side effects. This is an example of which of the following?
 Effect modification
 Selection bias
 Differential bias