Suppose that a newspaper published the following in a
boxed corner of the want ads section:
Should handgun control be abolished? You can call the shots if
you participate in our poll tonight. If yes, call 1-900-For-Guns.
If no, call 1-900-Ban-Guns. You will be charged 50¢ for the
first minute.
Why will the results of this poll be biased?
Those who respond will tend to be at the extremes in terms of both views. Those who are ambivalent or not sure are not as likely to respond. There is no option for undecided or unsure.
Not only that, they will have to have access to a phone and be willing to pay the toll. On the other hand, what will keep any person from voting multiple times?
It will also be biased, because it is limited to the readers of that particular publication.
I hope this helps. Thanks for asking.