As people entered a certain meeting, /I persons were selected at random; and each was asked either to name one of two political candidates he favored in a forthcoming election or to say " undecided" if he had no real preference. During the meeting, the people heard a speech on behalf · of one of the candidates. After the meeting, each of the same n persons was again asked to express his opinion. Describe a method for evaluating the effectiveness of the speaker.