Advice columnist Ann Landers once asked her readers with children to answer the following question:

Question

Advice columnist Ann Landers once asked her readers with children to answer the following question: “If you had it to do over again, would you have children?” Readers were invited to send a response to this question by mail. Of the approximately 10,000 responses Landers received, approximately 70% said “no.” The sample:
A) Is probably not representative because more than 10,000 people responded.
B) Has little bias because more than 10,000 people responded, yielding a very large sample
C) Is probably representative of all parents
D) Is probably not representative of all parents because people who feel very strongly about this issue are more likely to respond than people who do not.

Answer 1

Classic example of an invalid opinion poll because it is based on VOLUNTARY response sampling.  People who feel most strongly are the ones who are willing to take the time to respond.  So the poll is biased.  It is a form of self-selection bias (i.e., the respondents selected themselves), and this could have been eliminated by using random sampling.  The correct answer is (D).