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Evidence Synthesis

This guide provides an introduction to evidence synthesis as it is understood in evidence-based practice. clinical research and healthcare policy arenas.

Data Extraction

Once you have selected the relevant studies to be included in your systematic review, a standardized data extraction form or template will help ensure you obtain all relevant information from each study, allowing you to examine and compare results.

As with the study selection process, it's a good idea to have two reviewers complete the data extraction independently, and to have a plan for resolving disagreement

 

The Data Extraction Form

A data extraction form is essentially a template, tailored to fit the needs of your review, that you will fill out for each included study. The length and format of this form will vary based on factors such as the nature of your research question, the number of studies you are including, the types of studies you are including, and what types of analysis you are conducting.

How to develop a data extraction form for systematic reviews:

Determine what data items you need to collect.

The data that is relevant to the review will be different for every systematic review. Some categories of data that might be included:

  • Identification features (such as article title, author, citation, record number, type of publication, publication date, funding sources)
  • Methods
  • Participant characteristics
  • Intervention and Setting
  • Outcome/Results
  • Study quality factors
Determine the best way to collect that information.

Pre-specifying categories for form fields can help systematize the extraction process, and is especially useful when you plan on performing a quantitative analysis. When doing so, be sure to include, "other," "not reported," or "unclear" options.

At other times, it may make more sense to have an open field.

Select a tool to organize your data extraction.

Types of tools you might consider for your data extraction form:

  • Word Processors (e.g. Microsoft Word)
  • Spreadsheets (e.g. Microsoft Excel)
  • Surveys (e.g. Qualtrics)
  • Systematic Review tools (e.g. Covidence, RevMan, SRDR)
Run a pilot Try your form out on a sample set of studies and refine it as needed.

Examples of Data Extraction Forms, sometimes also called Evidence Tables, Evidence Synthesis Tables:

Williams, J.W., Slubicki, M.N., & Tweedy, D.S., Bradford, D.W., Trivedi, R.B., & Baker, D. (2009). Evidence Synthesis for Determining the Responsiveness of Depression Questionnaires and Optimal Treatment Duration for Antidepressant Medication [Internet]. Washington, DC: Department of Veterans Affairs (US). https://www.ncbi.nlm.nih.gov/books/NBK49047/table/appendixes.app5.t2/?report=objectonly (Evidence Table 2).

National Institute for Health and Care Excellence. (2012). Methods for the development of NICE public health guidance (third edition): Appendix K examples of evidence tables. https://www.nice.org.uk/process/pmg4/chapter/appendix-k-examples-of-evidence-tables#k2-example-of-evidence-table-for-qualitative-studies (this link includes example for both quantitative and qualitative tables).

Fineout-Overholt E, Melnyk BM, Stillwell SB, & Williamson KM. (2010). Evidence-based practice step by step. Critical appraisal of the evidence: part I: An introduction to gathering, evaluating, and recording the evidence...fifth in a series. AJN American Journal of Nursing, 110(7), 47–52. https://doi.org/10.1097/01.NAJ.0000383935.22721.9c 

http://download.lww.com/wolterskluwer_vitalstream_com/PermaLink/NCNJ/A/NCNJ_541_516_2011_01_13_DFGD_5161_SDC516.pdf 

(Evidence table example in the text of this article).