A researcher from Louisiana hypothesizes that people living out in the country display fewer avoidance strategies under stress than do people living in large cities. To test his hypothesis, he completes research with a group of participants from New Orleans and a group of participants from Church Point, Louisiana, and finds support for his hypothesis. However, he later learns that most of the participants from New Orleans were there during Hurricane Katrina. In this example, the experience of Hurricane Katrina is considered what type of variable?