Merchants in the locality responded to the local tax by reducing the price at which they sold cigarettes by 20 cents per pack. A substantial number of cigarette smokers in the locality who did not quit smoking during the campaign now smoke less than they did before it began. Smokers in the locality had incomes that on average were 25 percent lower than those of nonsmokers. Residents of the locality have not increased their use of other tobacco products such as snuff and chewing tobacco since the campaign went into effect.
One year ago a local government initiated an antismoking advertising campaign in local newspapers, which it financed by imposing a tax on cigarettes of 20 cents per pack. One year later, the number of people in the locality who smoke cigarettes had declined by 3 percent. Clearly, what was said in the advertisements had an effect, although a small one, on the number of people in the locality who smoke cigarettes.
Which one of the following, if true, most helps to strengthen the argument?