A University Wants to Know the Average Yearly Salary of Its

Essay

A university wants to know the average yearly salary of its alumni ten years after graduation. It has a list of all alumni who graduated ten years ago and writes to each one, asking for information about annual salary. Suppose that $40 \%$ of the alumni respond to this invitation.
-Would you consider the sample to be representative of the population, or is this sampling method biased? Explain.
A national survey of 12- to 17 -year-olds conducted in 2011 found that teens who spend time on a social networking site in a typical day are three times more likely to use alcohol than teens who do not spend time on a social networking site in a typical day. Headlines proclaimed that social networking causes drinking. A blogger responded that this conclusion is not justified and that age is a confounding variable.