A researcher does a study of a curriculum on mass transit. She assigns a group of students from an urban school in a large city to receive the curriculum. She assigns a school in a rural area to the comparison condition. She measures the results of a posttest on the curriculum and finds that the group who received the curriculum scores higher. Her colleague argues that there is a problem in concluding that the curriculum led to better performance because urban students will know more about mass transit than rural students even without the curriculum. Urban versus rural is what kind of variable