Selection criteria are important to determine in advance of screening records and are especially important for reviews that aim to minimize the chance of bias. 

Inclusion criteria: variables that are required for an article to be eligible for inclusion in a literature review.  Examples: experimental design, human subjects, English language, match of population/intervention/outcome

Exclusion criteria: variables that render an article inappropriate for inclusion. Exclusion criteria may not be as easy to identify in advance, but may become apparent with screening a test batch of records. Examples:healthy subjects (rather than patients), mixed age groups, funding from pharmaceutical industry.