marketer sends out a newsletter to one group that has the sent address of 'newsletter@companyX.com.$\text{'}$ This marketer sends out the same newsletter to another group that has the sent address of 'alex$\_$g@companyX.com.$\text{'}$ The researcher is attempting to determine the differences in response and open rates based on which address the newsletter is sent from. What is the marketer engaging in$?$