‘A set of predefined characteristics used to identify subjects who will be included in a research study’ (Velasco 2010). Inclusion and exclusion criteria are used to create the selection and eligibility criteria used to determine who can participate in a research project. Inclusion and exclusion criteria should be based on both methodological and ethical grounds. Poor choices of inclusion and exclusion criteria may unfairly distribute the harms and benefits of research, leading to exploitation of vulnerable populations or making it impossible 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 be investigated. (see also Exclusion Criteria)