Trina and Shannon study monkey behavior and are specifically interested in assessing how comfortable monkeys are around human observers. They individually conduct their observations on two groups of monkeys: one group that sees an observer wearing a mask and another group that sees an observer not wearing a mask. They want to see if masks impact how comfortable the monkeys are around the human observers. Every time Trina goes to observe her group without her mask on, she gives each of the monkeys a special treat before she observes them. What type of threat to internal validity might be going on in this scenario