Problem: The Web site www.twiigs.com allows you to vote on polls that interest you or to post one of your own. Onceyou have found a poll of interest, you just click on "Vote," and your response becomes part of the sample. One of the questions in July 2010 was "How many times have you been pulled over by the police?" Of the 780 people responding, 70% said "1-5 times." You can conclude that
(a) about 70% of Americans have been pulled over by the police "1-5 times."
(b) the poll uses voluntary response, so the results tell us little about the population of all adults.
(c) more people still need to vote on the question, as a larger sample is required to reduce bias.