In this step of the systematic review, you will develop your evidence tables, which give detailed information for each study (perhaps using a PICO framework as a guide), and summary tables, which give a high-level overview of the findings of your review. You can create evidence and summary tables to describe study characteristics, results, or both. These tables will help you determine which studies, if any, are eligible for quantitative synthesis.
Data extraction requires a lot of planning. We will review some of the tools you can use for data extraction, the types of information you will want to extract, and the options available in the systematic review software used here at UNC, Covidence.
The Cochrane Handbook and other studies strongly suggest at least two reviewers and extractors to reduce the number of errors.
You should plan to extract data that is relevant to answering the question posed in your systematic review. As mentioned previously, it may help to consult other similar systematic reviews to identify what data to collect. You should use your key questions and your eligibility criteria as a starting point. It can also help to think about your question in a framework such as PICO.
Helpful data may include:
If you plan to synthesize data, you will want to collect additional information such as sample sizes, effect sizes, dependent variables, reliability measures, pre-test data, post-test data, follow-up data, and statistical tests used.
Most systematic review software tools have data extraction functionality that can save you time and effort. Here at UNC, we use a systematic review software called Covidence. You can see a more complete list of options in the Systematic Review Toolbox.
Covidence allows you to
Covidence's new data extraction features are detailed in the "Data Extraction in Covidence" box at the end of this page.
You can also use spreadsheet or database software to create custom extraction forms. Spreadsheet functions such as drop-down menus and range checks can speed up the process and help prevent data entry errors. Relational databases (such as Microsoft Access) can help you extract information from different categories like citation details, demographics, participant selection, intervention, outcomes, etc.
RevMan offers collection forms for descriptive information on population, interventions, and outcomes, and quality assessments, as well as for data for analysis and forest plots. The form elements may not be changed, and data must be entered manually. RevMan is a free software download.
Survey or form tools can help you create custom forms with many different question types, such as multiple choice, drop downs, ranking, and more. Content from these tools can often be exported to spreadsheet or database software as well. Here at UNC we have access to the survey/form software Qualtrics.
In the past, people often used paper and pencil to record the data they extracted from articles. Handwritten extraction is less popular now due to widespread electronic tools. You can record extracted data in electronic tables or forms created in Microsoft Word or other word processing programs, but this process may take longer than many of our previously listed methods. If chosen, the electronic document or paper-and-pencil extraction methods should only be used for small reviews, as larger sets of articles may become unwieldy. These methods may also be more prone to errors in data entry than some of the more automated methods.