A college mailed a questionnaire to all 5432 of its alumni who graduated in the last 5 years. One of the questions was about the current annual incomes of these alumni. Only 1620 of these alumni returned the completed questionnaires, and 1240 of them answered that question. The current mean annual income of these 1240 respondents was $61,200.
a. Do you think $61,200 is likely to be an unbiased estimate of the current mean annual income of all 5432 alumni? If so, explain why.
b. If you think that $61,200 is probably a biased estimate of the current mean annual income of all 5432 alumni, what sources of systematic errors discussed in Section A.2.3 do you think are present here?
c. Do you expect the estimate of $61,200 to be above or below the current mean annual income of all 5432 alumni? Explain.