Selecting studies to include in the review
Cochrane Training resources: selecting studies and Covidence webinar (online tool for review production)
Cochrane Interactive Learning (CIL): module 4 - selecting studies and collecting data
| Standard | Rationale and elaboration | Resources | |
| C39 | Making inclusion decisions | Mandatory | |
| Use (at least) two people working independently to determine whether each study meets the eligibility criteria, and define in advance the process for resolving disagreements. | Duplicating the study selection process reduces both the risk of making mistakes and the possibility that selection is influenced by a single person’s biases. The inclusion decisions should be based on the full texts of potentially eligible studies when possible, usually after an initial screen of titles and abstracts. It is desirable, but not mandatory, that two people undertake this initial screening, working independently. | See Handbook Section 4.6.4 | |
| C40 | Excluding studies without useable data | Mandatory | |
| Include studies in the review irrespective of whether measured outcome data are reported in a ‘usable’ way. | Systematic reviews typically should seek to include all relevant participants who have been included in eligible study designs of the relevant interventions and had the outcomes of interest measured. Reviews must not exclude studies solely on the basis of reporting of the outcome data, since this may introduce bias due to selective outcome reporting and risk undermining the systematic review process. While such studies cannot be included in meta-analyses, the implications of their omission should be considered. Note that studies may legitimately be excluded because outcomes were not measured. Furthermore, issues may be different for adverse effects outcomes, since the pool of studies may be much larger and it can be difficult to assess whether such outcomes were measured. | See Handbook Section 4.6.3 | |
| C41 | Documenting decisions about records identified | Mandatory | |
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Document the selection process in sufficient detail to be able to complete a flow diagram and a table of ‘Characteristics of excluded studies’. |
Decisions should therefore be documented for all records identified by the search. Numbers of records are sufficient for exclusions based on initial screening of titles and abstracts. Broad categorizations are sufficient for records classed as potentially eligible during an initial screen. Studies listed in the table of ‘Characteristics of excluded studies’ should be those that a user might reasonably expect to find in the review. At least one explicit reason for their exclusion must be documented. Authors will need to decide for each review when to map records to studies (if multiple records refer to one study). Lists of included and excluded studies must be based on studies rather than records. | See Handbook Section 4.6.4 | |
| C42 | Collating multiple reports | Mandatory | |
| Collate multiple reports of the same study, so that each study, rather than each report, is the unit of interest in the review. | It is wrong to consider multiple reports of the same study as if they are multiple studies. Secondary reports of a study should not be discarded, however, since they may contain valuable information about the design and conduct. Review authors must choose and justify which report to use as a source for study results. | See Handbook Sections 4.6.2; 5.2.1 |