Extract data
After you’ve screened your search results, the next step in your review is to extract the necessary data from your included studies.
This will help you prepare to:
- assess the validity of the studies
- analyse and interpret the data.
To begin, decide what data elements you require for analysis. The below table is a sample of elements to consider. The full table can be viewed in Section 8 of the Cochrane Handbook.
Data
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Examples
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Source
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Study unique identifier (created by review author)
Citation and contact details
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Eligibility
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Reasons for inclusion or exclusion
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Methods
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The methodologies employed in the study, including the study design, duration, blinding, notes about bias, etc.
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Participants
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Description of eligible participants, including diagnostic criteria, acceptable co-morbidities, age, gender, setting or region
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Interventions
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Description of interventions and comparisons (if used), including control groups and method of delivery
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Outcomes
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Descriptions of outcomes, including definitions, units and methods of measurement and time points collected
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Results
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Data descriptions, including sample sizes and summary data
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Miscellaneous
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Key conclusions of the study authors
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Once you’ve decided which data elements are needed, use a data extraction form as a consistent way to record the information.
Appraise selected studies
Examine the studies for methodological rigour and signs of bias, which can lead to the study overstating or understating the true effect of outcomes.
Appraisal tools
Several appraisal tools can help you assess the publications and studies included in your systematic review.
Publication/study type
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Appraisal tool
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Systematic reviews
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Randomised controlled trials
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Non-randomised studies of the effects of interventions (NRSIs)
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Observational studies (cohort, case-control)
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Multiple study types
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Grey literature
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Avoid incomplete reporting
Incomplete reporting in a study can affect how you assess risk of bias.
Excluded or incomplete results have a bearing on whether you can trust the study as a whole, or if you can only trust specific outcomes.
Some ways to detect incomplete reporting include:
- using a study appraisal checklist
- comparing the methods to the result
- comparing the outcomes of a protocol (if available) to the published article
- asking the study authors for more information.
After appraisal, you might need to exclude further studies from your systematic review based on your eligibility criteria. Record your reasons for excluding studies as this will need to be reported in your review.