Defining your inclusion and exclusion criteria in advance is as important as defining your question and are determined by your research question.
These criteria determine which ‘subjects’ (studies) will be included in or excluded from the review and provide a safeguard against subjective selection bias by the reviewers.
Inclusion/exclusion criteria usually include:
- Outcome measure - (If you are looking for studies that report the results of a certain test, any study that doesn't report such results is then excluded from your review)
- Population - e.g. age/gender/ethnicity/co-morbidities and related conditions, etc.
- Publication type/date
- Geographical location
See: Cochrane Handbook for Systematic Reviews of Interventions, Chapter 5: Defining the review question and developing criteria for including studies.