23. A large study evaluating a national mass media smoking cessation campaign in the United States recruited participants using a ". ..random-digit-dial method, from February 5 through April 15, 2008, prior to the national launch of the. . . me- dia campaign" (Vallone et al., 2011, p. S39). Explain to a person who has never had a course in statistics (a) why this method of sampling might be used and (b) why it may be a problem if not everyone called agreed to be interviewed