‘A set of predefined definitions that is used to identify subjects who will not be included or who will have to withdraw from a research study after being included’ (Velasco 2010). Inclusion and exclusion criteria are used to determine who is eligible and can be selected to participate in a research project. Inclusion and exclusion criteria should be based on both methodological and ethical grounds (see also Diversity).

Poor choices of inclusion and exclusion criteria may unfairly distribute the harms and benefits of research, leading to exploitation of vulnerable populations or the inability for the benefits of research to be applied to particular populations.

Similar issues may arise in literature or systematic reviews where the choices of exclusion and inclusion criteria can inappropriately shape the results. Unexamined assumptions about a ‘hierarchy of methods’ are often responsible for this, rather than considering the fit between problems and the methods by which they might best be investigated. (See Inclusion Criteria)