Audrey, a senior thesis student, is interested in the relation between childhood residential mobility and adulthood socio-emotional outcomes. She predicts that moving from one home to another as a child might negatively influence trust as an adult. To test her claim, she asks participants to self-identify whether they moved often as a child or not. She then asks them to complete questionnaires about trust in friendships. Later, she compares responses of those who moved with those who did not. What type of measurement is Audrey using