We want to evaluate dog owners’ reactions to a new dog food product formulation that contains more vegetables. A promotional booth is set up at 10 dog events around the country. We distribute a one-day sample of the new more-vegetable formula to dog owners who come by the booth. If dog owners also provide their e-mail, they will be emailed a 20% off coupon for their first purchase. We measure the effectiveness of the more-vegetable formula by the number of coupons that are used to make purchases in stores. This is an example of an experiment designed to assess a ______ asymmetrical causal relationship.