n problems 13 through 15, explain why the results of the studies described are unreliable.

13)The TV columnist for a local newspaper invites readers to log on to a website to vote for their favorite TV newscaster.

14)A life insurance company wants to study the life expectancy of people born in 1950.The company's actuaries examine death certificates of people born in that year to determine how long they lived.

15)A congressman sent out questionnaires to 10,000 constituents to ask their opinions on a new health-care proposal.A total of 200 questionnaires were returned, and 70% of those responding supported the proposal.