You have been asked to evaluate the effectiveness of a new anti-smoking campaign in a local high school. You are able to collect pretest information on smoking rates before the campaign begins. You know that the entire school will be required to participate in the campaign and so random assignment is impossible. You are also aware that a reduction in smoking after the campaign could be caused by any number of other confounding variables. What can you do in this situation to enhance your ability to determine if the campaign was effective in reducing smoking