Now, let us assume that an ideal situation exists and that nobody in the city smokes. What will be the potential for preventing lung cancer through the use of the completely effective smoking cessation program that we wish to apply to the population of the city? The answer is zero; because there are no exposed people in the city, a program to eliminate the exposure would have no effect on the risk of lung cancer. Therefore, the spectrum of potential effect runs from a maximum (if everybody smokes) to zero (if nobody smokes). Of course, in reality, the answer is generally somewhere in between, because some members of the population smoke and some do not. The latter group (all nonsmokers) clearly will not benefit from a smoking cessation program, regardless of how effective it is.