Entering students at the University of Washington were asked to complete a survey in the fall of 1999. Out of about 4,000 entering students, roughly 2,000 completed the survey. The sample of 2,000 students who completed the survey were found to differ in a variety of ways (age, ethnicity, SAT scores) from the population of 4,000 entering students. Explain why our formulas for standard deviation of sample proportion or sample mean, which require independent selections, do not apply in this situation.