Sampling Methods for Political Polling - AAPOR
www.aapor.org
Non-probability sampling methods do not share this feature that everyone in a population has a chance of being selected and researchers know exactly what that chance is. Participants are typically not selected at random to be included in the sample but rather come to be included by other means, for instance because they volunteer, a person’s chance of being in the sample is unknown. For example, in an opt-in sample a person accepts an invitation to complete a survey that is offered to all visitors to a website. The chance of that person visiting that website and then choosing to participate in the survey cannot be known. One serious consequence is that only certain types of people may choose to opt into the survey and they may be different than those who do not in ways that could potentially bias the final results.
With non-probability samples is there is no simple way to calculate the “margin of error;” instead, estimates of the likely error must be based on a statistical models. As a result, AAPOR has cautioned that it may be misleading to report a margin of sampling error for surveys based on non-probability samples.