For this midterm exam question, Walmart would like to increase the number of customers using their online order and pickup service. They have decided to run an experiment with 600 randomly selected individuals who live within 10 miles of a Walmart store. They will randomly assign 200 individuals to receive a mailing with free order pickup for one month. A second group of randomly assigned individuals will receive a mailing with a one-time 50% off online order and pickup. The third group of randomly assigned individuals will receive the standard Walmart mailing with no coupons. After three months, they will compare the number of individuals in each group who used the online order and pickup service at least once. Name one possible lurking variable for this experiment.