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Evidence Synthesis & Systematic Review Research

This guide provides an introduction to evidence synthesis research methods.

Synthesis Overview

This is the point in the work where the main findings of the project are integrated and presented. The idea is that the synthesis work goes beyond summarizing each individual study and instead focuses on generating an understanding of what the integrated body of research offers. Methods for synthesizing the data from the included studies will depend on the type of evidence synthesis conducted (e.g. narrative review, evidence map, systematic review, etc.) and on the type of data collected (e.g. qualitative, quantitative, mixed).

Quantitative Synthesis

A common synthesis technique for quantitative data derived from "studies that have empirically tested the same
hypothesis" (Siddaway et al., 2019) is meta-analysis, 'the statistical combination of results from two or more separate studies, ultimately to yield an overall statistic (together with its confidence interval) that summarizes the effectiveness of an experimental intervention compared with a comparator intervention' (Deeks et al., 2024). Evidence syntheses focused on studies with quantitative data (e.g. systematic review, rapid review) are often extended to include meta-analysis (of effect estimates).

Siddaway et al. (2019) share that meta-analysis would be appropriate when a collection of studies

  • report quantitative results (data) rather than qualitative findings or theory;
  • examine the same or similar constructs/relationships;
  • are derived from similar research designs;
  • report the simple relationships between two variables (bivariate relationships, zero-order correlations, single-degree-of-freedom contrasts), rather than relationships that have been adjusted for the effect of additional variables (e.g., partial or multivariate effects);
  • and have results that can be configured as standardized effect sizes (Borenstein et al. 2009)

However, there are a variety of reasons that meta-analysis may not be possible with quantitative data including (but not limited to) the following: limited evidence; incompletely reported outcome/effect estimates, or different effect measures used across studies; bias in the evidence; clinical and methodological diversity; or statistical heterogeneity. Some acceptable, though not preferable techniques, for addressing such situations include: summarizing effect estimates; combining P value; and vote counting based on direction of effect (NOT vote counting based on statistical significance). Chapter 12 and Table 12.2.a of the Cochrane Handbook for Systematic Reviews of Interventions provide a more detailed overview of the reasons for using alternate quantitative synthesis and the limitations in doing so, and should be thoroughly reviewed (McKenzie et al., 2019).

 

OSU Libraries has numerous resources available for learning more about meta-analysis.

 

References:

  • Borenstein M., Hedges L.V., Higgins J.P.T., Rothstein H.R. (2009). Introduction to Meta-Analysis. New York: Wiley
  • Deeks JJ, Higgins JPT, Altman DG, McKenzie JE, Veroniki AA (editors). Chapter 10: Analysing data and undertaking meta-analyses [last updated November 2024]. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.5. Cochrane, 2024. Available from cochrane.org/handbook.
  • McKenzie JE, Brennan SE. Chapter 12: Synthesizing and presenting findings using other methods [last updated October 2019]. In: Higgins JPT, Thomas J, Chandler J, Cumpston M, Li T, Page MJ, Welch VA (editors). Cochrane Handbook for Systematic Reviews of Interventions version 6.5. Cochrane, 2024. Available from cochrane.org/handbook.
  • Siddaway, A. P., Wood, A. M., & Hedges, L. V. (2019). How to do a systematic review: A best practice guide for conducting and reporting narrative reviews, meta-analyses, and meta-syntheses. Annual Review of Psychology, 70(1), 747–770. https://doi.org/10.1146/annurev-psych-010418-102803

Qualitative Synthesis

Synthesis of qualitative data utilizes a variety of methods. Handbook-style guidance on determining when to use various methods is actively underway in a collaborative project between Cochrane Qualitative and Implementation Methods Group and members of the Campbell Qualitative Evidence Synthesis working group who are working to produce the Cochrane-Campbell Handbook for Qualitative Evidence Synthesis (completed publication due 2026).

Qualitative synthesis could take some of the following forms:

  • critical interpretive synthesis "constructs a critical analysis and synthesis of a complex body of literature...[taking an] inductive
    approach in order to create an overarching theory by synthesising theoretical categories extracted from the available qualitative and quantitative evidence on the topic or phenomenon of interest. An important feature of critical interpretive synthesis is that it draws on all forms of evidence in included studies and not just the ‘themes’ in included evidence." (Noyes et al., 2025)
  • framework synthesis "uses a selected, adapted or created theory, framework or model to guide data extraction, analysis and interpretation of findings from primary research." (Brunton et al., 2024)
  • meta-aggregation is "specifically for the synthesis of qualitative studies addressing practice-based questions." (Noyes et al., 2025)
  • meta-ethnography is an "explicitly theory building approach to the synthesis of qualitative evidence, with synthesis drawing on the interpretations, concepts or theory generated by authors of the included studies (known as ‘second order constructs’) (Garside et al, 2023)
  • meta-narrative synthesis "compares and contrasts the storylines about similar phenomena of interest developed by different disciplines and traditions (such as medicine, nursing and patients or government, policy-implementers and citizens)" (Noyes et al., 2025)
  • narrative review "synthesizes results of individual quantitative studies with no reference to statistical significance of the finding" (Siddaway et al., 2019) for situations when the quantitative studies included have used "diverse methodologies, or have examined different theoretical conceptualizations, constructs, and/or relationships" (Baumeister, 2013 in Siddaway et al, 2019).
  • qualitative comparative analysis "synthesises codified qualitative and quantitative data to identify critical intervention and / or contextual elements that combine in non-additive or non-linear ways to produce a successful outcome (Ragin, 1987 in Sutcliffe et al., 2023).
  • realist synthesis "extend[s] explanations beyond “what works” to factor in considerations of meaning, context and implementation...draw[ing] upon the explanatory power offered by qualitative evidence but...seek[ing] to bring this together with quantitative evidence within the realist review" (Booth et al., 2024).
  • thematic synthesis  to move through the processes of "coding of text 'line-by-line'; the development of 'descriptive themes'; and the generation of 'analytical themes'" (Thomas & Harden, 2008).

 

References: