Another SLOP survey:
Quote:
A good poll story begins with a good poll. At the heart of a good poll is a randomly selected representative sample of the target population. Unfortunately, bad polls and bad samples are everywhere, and stories based on those flawed polls find their way on air or into print with dismaying frequency. One reason is that it’s hard and sometimes prohibitively expensive to collect a random or representative sample. Instead, some researchers use convenience samples.
One common type of convenience sample produces surveys that researchers call self-selected opinion polls, or SLOP surveys. As the name suggests, the sample in a SLOP survey is not selected randomly. Instead, individuals choose whether to participate. Margin of sampling error cannot be estimated for a SLOP poll, no matter how large...
...Researchers have learned, often to their great embarrassment, that these types of samples often produce flawed results. Respondents who volunteer to participate in such surveys tend to be more extreme or otherwise very different in their views than those who do not. In no way can they be said to be representative of the population, so the survey results cannot be used to say anything useful about a target population.
http://www.aapor.org/badandworsesamples
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You do realize that more than half of facebook members are not even US voters.