Evaluate the following generalization(s), identifying sample, population, attribute of interest, and the extent to which the claims involved are knowable. Consider carefully the size and diversification of the sample and the extent to which the population differs or may differ from the sample; remember, what's important is that the sample be representative.A poll of fifty weight lifters at a southern California gym determined that thirty-three payed close attention to their diets as well as to their exercise. Of those thirty-three, twenty-five (50 percent of the original fifty) made it a point to eat more than the minimum daily amount of protein for large adults, and twenty (40 percent of the original fifty) took vitamin pills and other dietary supplements. The chain of health-food stores

Question

that took the poll concluded that weight lifters constitute a substantial market for its products, since it is likely that 40 percent of all weight lifters across the country take vitamin pills and supplements and that an additional 10 percent are at least highly conscious of their diets.

What will be an ideal response?

Answer 1

The health-food chain had best not invest too much in attracting this new market. The first flaw in the survey is technical: the sample is too small to give a very detailed picture of weight lifters' habits. Even if nothing else were wrong with the survey, strong confidence (95 percent) would be justified only in the claim that from about 36 to 64 percent of the weight lifters nationwide make sure to eat more than the usual amount of protein-not a very precise conclusion (see the error margin table [Table 10-1] in the text, page 360). A more important flaw in the survey is that the interviews are confined to clients of only one gym. Information is often passed around among people who frequent the same establishments, and there may be trends or fads or a particular bit of useful information that is current at one gym but not in others. The sample is biased, in other words.