In a laboratory, smokers are asked to "drive" using a computerized driving simulator equipped with a stick shift and a gas pedal. The object is to maximize the distance covered by driving as fast as possible on a winding road while avoiding rear-end collisions. Some of the participants smoke a real cigarette immediately before climbing into the driver's seat. Others smoke a fake cigarette without nicotine. You are interested in comparing how many collisions the two groups have. In this study, the participants who smoke a cigarette without nicotine comprise the