When making purchases with cash, some people drop their pennies in the “penny cup” next to the cash register and some don’t. A social psychologist wondered if those who did were more altruistic. She obtained a sample of people who dropped their pennies into penny cups and a sample who didn’t. To each person, she administered an interval level measure of altruism. What statistical test should she use to see if the groups differ on the mean level of altruism?