Standard statistical literature has been developed almost entirely in terms of independent observations obtained by simple random sampling (SRS). The statistical tests and confidence intervals one finds in textbooks and in journal articles are based on the assumption that the sample observations were selected independently, at random. Although this assumption either appears in "fine print" or not at all, it constitutes the basis of all confidence intervals and tests of hypotheses, their validity is based on it. The ubiquity of the SRS assumption in statistical theory can be explained by the basic nature of SRS and by its facilitation of interesting theoretical results. On the other hand, most social research, especially survey work, is in fact carried out by means of complex sample designs. Simple random selection of human populations is a rare phenomenon, limited to small and confined populations. Socially important human populations are usually large and widely scattered, so that a simple random selection of them would prove in most cases to be uneconomical and impractical.