This is when the way the people who are considered in a study uninitentionally includes some kinds of people more than others and thus induces a bias in the way resuts are interpretted. This may occur due to sampling bias in the way people are initially selected, or other effects such as self-selection of participants in study, or participants dropping out. Note that this affects both qualitative and quantitative analysis.
Links:
catalogofbias.org: Selection Bias in OUP Catalog of Bias- ebmh.bmj.com: Appraising the evidence: what is selection bias?
- sciencedirect.com: Selection Bias
- Wikipedia: Selection Bias