Self-selection bias
Self-selection bias (a.k.a. selection threat, self-selection effect) is a problem where the participants of an evaluation or research project join because they choose to themselves (rather than randomly being selected by the evaluator). For example, a web-based survey put out on Twitter about people’s opinions on pet cats will likely attract people who are already interested in cats. As a result, the sample may be biased and not representative of the whole population. Instead, the sample represents cat-lovers.