According to Cochrane, the creation of clear inclusion and exclusion criteria is integral to any review process. Understanding what terms and phrases apply to your search and which ones do not, among all members of the research team is a crucial step in the review process.

Defining eligibility criteria will streamline the searching, screening, and review process. This step should take place after defining your research question and before you start crafting your search strategies.

This is a key feature that distinguishes knowledge syntheses from non-methods driven literature reviews.

Overall, your inclusion and eligibility criteria are a central feature of your protocol and 'a priori' design for your systematic review. This criteria guides which studies are selected for inclusion in your study during the review steps for: 1) titles and abstract and 2) full-text.

It is determined during the development of your protocol (plan) for the review. Some knowledge syntheses allow for the modification of eligibility criteria at later stages (e.g. scoping reviews). Any changes to your criteria, different than those in your protocol, should be clearly reported and justified in your manuscript.

Eligibility criteria should be precisely defined and all key elements of the question considered, for example: population, problem, intervention, issue, exposure of interest, study design, publication type. 

How ‘broad’ or ‘narrow’ your criteria is determined by your research question.

Your research defines what will be included in the search and what will be excluded before creating the search strategy. Doing so will help clarify what exactly your research question aims to uncover.

(Source: The University of Melbourne, 2020)