Note: Erratum was posted in October 2022 to correct a scientific equation. Refer to page iv of the full report for more information.
For related information, go to the Series Overview, Methods Guide for Effectiveness and Comparative Effectiveness Reviews.
The purpose of this document is to consolidate and update quantitative synthesis guidance provided in three previous methods guides.1-3 We focus primarily on comparative effectiveness reviews (CERs), which are systematic reviews that compare the effectiveness and harms of alternative clinical options, and aim to help clinicians, policy makers, and patients make informed treatment choices. We focus on interventional studies and do not address diagnostic studies, individual patient level analysis, or observational studies, which are addressed elsewhere.4
Quantitative synthesis, or meta-analysis, is often essential for CERs to provide scientifically rigorous summary information. Quantitative synthesis should be conducted in a transparent and consistent way with methodologies reported explicitly. This guide provides practical recommendations on conducting synthesis. The guide is not meant to be a textbook on meta-analysis nor is it a comprehensive review of methods, but rather it is intended to provide a consistent approach for situations and decisions that are commonly faced by Evidence-based Practice Centers (EPCs). The goal is to describe choices as explicitly as possible and in the context of EPC requirements, with an appropriate degree of confidence.
EPC investigators are encouraged to follow these recommendations but may choose to use alternative methods if deemed necessary after discussion with their AHRQ project officer. If alternative methods are used, investigators are required to provide a rationale for their choices, and if appropriate, to state the strengths and limitations of the chosen methods in order to promote consistency, transparency, and learning. In addition, several steps in meta-analysis require subjective judgment, such as when combining studies or incorporating indirect evidence. For each subjective decision, investigators should fully explain how the decision was reached.
This guide addresses issues in the order that they are usually encountered in a synthesis, though we acknowledge that the process is not always linear. We first consider the decision of whether or not to combine studies quantitatively. The next chapter addresses how to extract and utilize data from individual studies to construct effect sizes, followed by a chapter on statistical model choice. The fourth chapter considers quantifying and exploring heterogeneity. The fifth describes an indirect evidence technique that has not been included in previous guidance—network meta-analysis, also known as mixed treatment comparisons. The final section in the report lays out future research suggestions.
This guide was developed by a workgroup comprised of members from across the EPCs, as well as from the Scientific Resource Center (SRC) of the AHRQ Effective Healthcare Program. Through surveys and discussions among AHRQ, Directors of EPCs, the Scientific Resource Center, and the Methods Steering Committee, quantitative synthesis was identified as a high-priority methods topic and a need was identified to update the original guidance.1,5 Once confirmed as a Methods Workgroup, the SRC solicited EPC workgroup volunteers, particularly those with quantitative methods expertise, including statisticians, librarians, thought leaders, and methodologists. Charged by AHRQ to update current guidance, the workgroup consisted of members from eight of 13 EPCs, the SRC, and AHRQ, and commenced in the fall of 2015. We conducted regular workgroup teleconference calls over the course of 14 months to discuss project direction and scope, assign and coordinate tasks, collect and analyze data, and discuss and edit draft documents. After constructing a draft table of contents, we surveyed all EPCs to ensure no topics of interest were missing.
The initial teleconference meeting was used to outline the draft, discuss the timeline, and agree upon a method for reaching consensus as described below. The larger workgroup then was split into subgroups each taking responsibility for a different chapter. The larger group participated in biweekly discussions via teleconference and email communication. Subgroups communicated separately (in addition to the larger meetings) to coordinate tasks, discuss the literature review results, and draft their respective chapters. Later, chapter drafts were combined into a larger document for workgroup review and discussion on the bi-weekly calls.
Literature Search and Review
A medical research librarian worked with each subgroup to identify a relevant search strategy for each chapter, and then combined these strategies into one overall search conducted for all chapters combined. The librarian conducted the search on the AHRQ SRC Methods Library, a bibliographic database curated by the SRC currently containing more than 16,000 citations of methodological works for systematic reviews and comparative effectiveness reviews, using descriptor and keyword strategies to identify quantitative synthesis methods research publications (descriptor search=all quantitative synthesis descriptors, and the keyword search=quantitative synthesis, meta-anal*, metaanal*, meta-regression in [anywhere field]). Search results were limited to English language and 2009 and later to capture citations published since AHRQ's previous methods guidance on quantitative synthesis. Additional articles were identified from recent systematic reviews, reference lists of reviews and editorials, and through the expert review process.
The search yielded 1,358 titles and abstracts which were reviewed by all workgroup members using ABSTRACKR software (available at http://abstrackr.cebm.brown.edu). Each subgroup separately identified articles relevant to their own chapter. Abstract review was done by single review, investigators included anything that could be potentially relevant. Each subgroup decided separately on final inclusion/exclusion based on full text articles.
Consensus and Recommendations
Reaching consensus if possible is of great importance for AHRQ methods guidance. The workgroup recognized this importance in its first meeting and agreed on a process for informal consensus and conflict resolution. Disagreements were thoroughly discussed and if possible, consensus was reached. If consensus was not reached, analytic options are discussed in the text. We did not employ a formal voting procedure to assess consensus.
A summary of the workgroup's key conclusions and recommendations was circulated for comment by EPC Directors and AHRQ officers at a biannual EPC Director's meeting in October 2016. In addition, a full draft was circulated to EPC Directors and AHRQ officers prior to peer review, and the manuscript was made available for public review. All comments have been considered by the team in the final preparation of this report.
Morton SC, Murad MH, O'Connor E, Lee CS, Booth M, Vandermeer BW, Snowden JM, D'Anci KE, Fu R, Gartlehner G, Wang Z, Steele DW. Quantitative Synthesis—An Update. Methods Guide for Comparative Effectiveness Reviews. (Prepared by the Scientific Resource Center under Contract No. 290-2012-0004-C). AHRQ Publication No. 18-EHC007-EF. Rockville, MD: Agency for Healthcare Research and Quality; February 2018. Posted final reports are located on the Effective Health Care Program search page. https://doi.org/10.23970/AHRQEPCMETHGUIDE3.
- Fu R, Gartlehner G, Grant M, et al. Conducting Quantitative Synthesis When Comparing Medical Interventions: AHRQ and the Effective Health Care Program Agency for Healthcare Research and Quality. Rockville, MD: 2010.
- Lau J, Terrin N, Fu R. Expanded Guidance on Selected Quantitative Synthesis Topics Agency for Healthcare Research and Quality. Rockville, MD: 2013.
- Lau J, Chang S, Berkman N, et al. EPC Response to IOM Standards for Systematic Reviews Agency for Healthcare Research and Quality. Rockville, MD: 201
- Chou R, Aronson N, Atkins D, et al. AHRQ series paper 4: assessing harms when comparing medical interventions: AHRQ and the effective health-care program. J Clin Epidemiol. 2010;63(5):502-12. PMID: 18823754 http://dx.doi.org/10.1016/j.jclinepi.2008.06.007
- Fu R, Vandermeer BW, Shamliyan TA, et al. Handling Continuous Outcomes in Quantitative Synthesis Agency for Healthcare Research and Quality. Rockville, MD: 2013.