By highlighting differences in survey methodology, this article at the Michigan State University site sheds light on why polling data does not always reflect the truth. It is rare for any poll to actually ask questions of every person in a particular group. Usually only a few members of the group are questioned as a sample of the population. Many factors can keep a poll from being a perfect indicator of how all of the people in the population think, feel, or behave. If we use the phone book to select 500 names at random to call, we will never question people with unlisted phone numbers or people who do not have a phone.
Research indicates that most phones are answered by women or older people.
A poll of 500 registered voters might not include anyone who will actually showup and cast a vote. Almost any question will bias the answer to some degree. Research indicates that the order in which the names are listed in the question will make a difference in the answer a person gives. Now that most Americans work outside the home, there is a very small window of time when pollsters can reach people by phone. The person who answers may be tired after a day's work, not to mention frustrated by a large number of unsolicited telephone calls.