Harry S. wanted to quit smoking. So when he saw an ad for a study on ways to break the habit, he called and offered to be part of it. The study was at a local university, where Harry and the other volunteers-all of them people who wanted to stop smoking-were divided into three groups. The volunteers in Group A received nicotine gum. (Nicotine is in tobacco. It is what gives smokers the good feeling they get from smoking.) When the smokers felt the need for a cigarette, they could chew a piece of this gum instead. It would give them the nicotine they were used to. The volunteers in Group B, including Harry, got some gum, too. It was just plain chewing gum, but the volunteers did not know that. The people in Group C got nothing. (A group like this in a study is called the control group.)