Bias that occurs when the people who were unable to be reached or unwilling to participate in a sample have substantially different opinions than the people who were included in the sample, resulting in a misrepresentation of the population.
Participation Bias
Bias that occurs when participation in a study is voluntary. People who feel strongly may be the only participants.
Response Bias
Bias that occurs when either (1) the question is poorly worded so that certain responses are over-represented, or (2) the respondent is confused by the question or feel like they should lie due to the sensitive nature of the question.