A categorical interacting variable defines subgroups for which the effect of the explanatory variable on the outcome variable differs. For instance, as explained in the text, for the nicotine patch experiment described in given Case Study, one interacting variable was whether or not there were other smokers at home. When the difference in subgroups is thought to hold for the population, the subgroup variable is sometimes called an effect modifier. Explain why this term makes sense, using the context of the smokers at home in the nicotine experiment. For instance, what "effect" is being modified?

Case Study
Quitting Smoking with Nicotine Patches
There is no longer any doubt that smoking cigarettes is hazardous to your health and the health of those around you. Yet for someone who is addicted to smoking, quitting is no simple matter. One promising technique is to apply a patch to the skin that dispenses nicotine into the blood. In fact, these nicotine patches have become one of the most frequently prescribed medications in the United States. To test the effectiveness of these patches on the cessation of smoking, Dr. Richard Hurt and his colleagues (1994) recruited 240 smokers at Mayo Clinics in Rochester, Minnesota; Jacksonville, Florida; and Scottsdale, Arizona.

Volunteers were required to be between the ages of 20 and 65, have an expired carbon monoxide level of 10 ppm or greater (showing that they were indeed smokers), be in good health, have a history of smoking at least 20 cigarettes per day for the past year, and be motivated to quit. Volunteers were randomly assigned to receive either 22-mg nicotine patches or placebo patches for 8 weeks. They were also provided with an intervention program recommended by the National Cancer Institute, in which they received counseling before, during, and for many months after the 8-week period of wearing the patches.

After the 8-week period of patch use, almost half (46%) of the nicotine group had quit smoking, while only one-fifth (20%) of the placebo group had done so. Having quit was defined as "self-reported abstinence (not even a puff) since the last visit and an expired air carbon monoxide level of 8 ppm or less" (p. 596). After a year, rates in both groups had declined, but the group that had received the nicotine patch still had a higher percentage who had successfully quit than did the placebo group, 27.5% versus 14.2%.

The study was double-blind, so neither the participants nor the nurses taking the measurements knew who had received the active nicotine patches. The study was funded by a grant from Lederle Laboratories and was published in the Journal of the American Medical Association. Because this was a well-designed randomized experiment, it is likely that the nicotine patches actually caused the additional percentage who had quit in the nicotine patch group compared with the control group.