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If, in an intervention study, an investigator can't describe a formal decision rule that he or she used to select subjects or assign treatments, then there is a risk of selection bias. Common types of selection bias in surveys include differences between volunteers vs randomly selected subjects, and responders vs non-responders. Another important source of selection bias arises from differences in access to extension activities and technical advice. For example, prawn farms that have regular input from trained specialists are very unlikely to be representative of all prawn farms.