In this argument, the author conclude that the change of replacing of butter with margarine in the southwestern United States has had little impact on their customers. This maybe true as the author assumes that there is few complaints from the customers and when the servers replace the butter they did not complain. But only resting on such evidence and without further information, the author's statement is inconvincible.
First of all, the author claims that there are an average of 98 percent of people are happy with the replacing for he/she assumes that the people who did not complain would be happy about the change. But this assumption is unwarranted for it is a false dilemma. The customers who did not complain does not indicate that they are satisfied with the replacing. They may just did not want to complain for it may be a long and complicated process for a complaint. And maybe the complaints would not be addicted at all. They might just choose another restaurants but they did not claim that. If the author wants to draw a firmly conclusion, he/she should take a detail survey about what the customers really think about.
Secondly, the author allege that the customers can not distinguish butter from margarine for the evidence that servers had reported few complaints from the customers and when given the margarine instead, they did not say anything. But in fact, there could be many alternative explanations for these phenomenon. For example the author does not give a convincible evidence that the servers did not lie for they are afraid of reporting the complaints because if there are too much dissatisfaction from the customers, they may be punished for bad services.
What's more, it is common sense for people to tell butter between margarine, so the scant of the customers’ complaints can also be a silent protest of the replacing. For they did not want to bother themselves when having dinner outside. But they would not come to this restaurant again and they would tell their friends for the replacing. Because of lacking such evidence about the actual attitudes of customers, the conclusion drawn is unwarranted.
To sum up, though it may be true that the customers really care few about the replacing and there has little impact on it. But just resting on such evidence and without the consideration of alternative explanations, the arguer can not come to this conclusion.