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. Other smoke a fake cigarette without nicotine. You are interested in comparing how many collisions the two groups have what us the independent variable and what is the dependent variable?