Suppose that random samples of Caucasian-American and Chinese-American individuals are obtained from patient records of doctors participating in a study to compare blood pressures of the two populations. Suppose that the individuals selected are asked whether they want to participate and that some decline. The study is conducted only on those that volunteer to participate, and a compar- ison of the distributions of blood pressures is conducted. Where does this study fit in Display 1.5? What assumption would be necessary to allow inferences to be made to the sampled populations?