At the beginning of large systematic reviews, researchers discuss and develop a series of inclusion and exclusion criteria to fit in with their review question and/or the brief provided by whoever is funding the project.
Systematic reviews often exclude studies if they do not conform to specific study designs, are not written in English or within a certain time frame. As a researcher, you should be cautious of any bias you might introduce into the review by adding certain inclusion or exclusion criteria. For example: limiting to studies in English may miss important studies published in other languages.
All decisions to include or exclude certain studies or groups of studies should be documented in the methods section of the research proposal/protocol - this way it can be demonstrated that a systematic process has been followed.
In large systematic reviews, the inclusion/exclusion criteria are applied to all the studies retrieved by the literature search. At this stage, the decisions are usually made using the titles and abstracts of the articles; those that are clearly irrelevant can be excluded.
Full text papers are obtained for the remaining articles and the criteria are applied again. Those that meet the criteria are included in the review (although sometimes if too many papers are obtained, the question and criteria are refined and the process repeated). This process is represented by the following flow diagram. (See PRISMA Flow Diagram)
Key Points Regarding Study Selection
Section 1.3.2. Process for Study selection (http://www.york.ac.uk/inst/crd/pdf/Systematic_Reviews.pdf, actual page #35)