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Table 1 Characteristics of a preliminary list of existing knowledge synthesis methods

From: What is the most appropriate knowledge synthesis method to conduct a review? Protocol for a scoping review

Knowledge synthesis method Type of evidence Description
Bayesian meta-analysis
(Sutton, AJ, 2001; Roberts KA, 2002) [18, 19]
Mixed (qualitative and quantitative) A method used in meta-analysis to offer flexibility in handling data from diverse study types (i.e., the integration qualitative and quantitative forms of evidence). It allows qualitative evidence to contribute to meta-analysis by identifying variables to be included and providing evidence about effect sizes (qualitative evidence gets converted into quantitative form); and helps to ensure that meta-analyses more properly reflect the diversity of evidence at primary level – it recognizes the fact that evidence from multiple sources usually needs to be combined to inform policy decisions.
Content analysis
(Stemler S, 2001) [20]
Qualitative A technique for categorising data and determining the frequencies of these categories. It differs from more ‘qualitative’ methods in that it requires categorization to be sufficiently precise to allow multiple coders to achieve the same results, it relies on the systematic application of rules, and it tends to draw on the concepts of validity and reliability. Text is condensed into fewer content-related categories.
Critical interpretive synthesis
(Dixon-Woods, 2006) [21]
Mixed Developed from meta-ethnography, it is an approach to the entire process of a review rather than just the synthesis component. It uses an iterative approach to refining the research question, the searching and selection of articles from the literature, and defining and applying codes and categories.
Cross-design synthesis
(Droitcour J, 1993) [22]
Mixed A form of meta-analysis, which allows the mixing of different quantitative research designs (e.g. randomized controlled trials and observational studies) and the pooling of evidence using modeling to estimate a ‘true’ effect of a policy or programme, conditional on both the design of the study and the characteristics of the relevant population
Ecological triangulation
(Banning, date unknown) [23]
Qualitative Uses the concept of triangulation, in which phenomena are studied from a variety of vantage points. The method ‘unpicks’ the mutually interdependent relationships between behaviour, persons, and environments, and requires ‘ecological sentences’ to be formulated during synthesis: “With this intervention, these outcomes occur with these population foci and within these ages with these genders… and these ethnicities in these settings”.
Framework synthesis
(Pope, 2000; Brunton, 2006) [24, 25]
Qualitative Offers a highly structured approach to organizing and analysing data (i.e., indexing using numerical codes, rearranging data into charts, etc) to handle the large volume of information resulting from qualitative research. It’s distinct from other methods in that it utilises an ‘a priori’ framework informed by background material and team discussions to extract and synthesize findings (i.e., a deductive approach). The ‘synthetic’ product may be expressed in the form of a chart for each key dimension, which can be used to map the nature and range of the concept under study.
Grounded theory
(Strauss & Corbin, 1998) [26]
Qualitative A primary research approach used as a method for qualitative sampling, data collection and analysis. It offers the ‘constant comparative method’ (the most widely used element of grounded theory) to be used to identify patterns and iterations in primary data. It is an inductive approach to analysis, allowing the theory to emerge from the data.
Interpretive Synthesis/Integrative synthesis
(Noblit and Hare (1988) [27]
Qualitative Noblit and Hare (1988) distinguish between the approaches of ‘interpretive’
and ‘integrative’ forms of synthesis which can be described as exploring the nature of the synthesis rather than its application. Interpretive synthesis combines evidence with an intent to develop new concepts and theories (interpretations).
(Noblit G & Hare R., 1988) [27]
Qualitative A novel synthesis method aimed to uncover a new theory to explain the range of research findings encountered. It is a way of re-analysing and comparing the texts of published studies (rather than the original data of each) to produce a new interpretation. The approach involves induction and interpretation in which separate parts are brought together to forma a “whole” (i.e., looking for new theory or ‘line of argument’ to explain all the studies) so that the result is greater than the sum of its parts. The product is the translation of studies into one another, which encourages the researcher to understand and transfer ideas, concepts and metaphors across different studies.
(Weed, 2005) [28]
Qualitative A method that follows an ideographic rather than pre-determined approach to the development of the following components: exclusion criteria, a focus on meaning in context, interpretations as raw data for synthesis, an iterative approach to the theoretical sampling of studies for synthesis, and a transparent audit trail demonstrating the trustworthiness of the synthesis
(Greenhalgh,2005) [29]
Qualitative A method developed from the need to synthesize evidence to inform complex policy-making questions, and involves looking across different paradigms/research traditions to uncover their ‘unfolding storyline” resulting in maps of ‘meta-narratives’ from which dimensions or themes can be revealed and distilled for the synthesis phase of the review.
(Paterson BL, 2001) [30]
Qualitative A multi-faceted, interpretive approach to synthesis developed to study the experiences of adults living with a chronic illness, and consists of 3 components to be done prior to synthesis: meta-data-analysis, meta-method, and meta-theory. Collectively, these create a new interpretation accounting for the results of all three elements of analysis.
(Sandelowski M, 2003) [31]
Qualitative A quantitatively oriented summary of qualitative findings (as opposed to data being transformed) developed to accommodate the distinctive features of qualitative surveys. The approach includes the extraction, grouping, and formatting of findings, and the calculation of frequency and intensity effect sizes, which can be used to produce mixed research syntheses and to conduct ‘posteriori’ analyses of the relationship between reports and findings. Meta-summaries can serve as a basis for a further synthesis.
(Sandelowski M, 1997) [32]
Qualitative A method developed in response to concerns about the relevance and utility of qualitative research, and involves combining separate elements to form a coherent whole using a process of logical deduction. Its aims are to portray an accurate interpretation of a phenomenon and to compare and contrast the constructs of individual studies to reach consensus on a new construction of that phenomenon. It involves: identifying findings, grouping findings into categories and grouping categories into synthesised findings.
Mixed studies review
(Pluye, 2005, Pluye 2009; Sandelowski M, 2003 book) [3335]
Mixed A literature review that simultaneously examines qualitative, quantitative, and mixed methods primary studies to provide a greater understanding of a health issue than one type of research approach alone (including the process of searching, analysis and study quality appraisal).
Narrative review / Narrative summary
(Dixon-Woods M, 2005) [36]
Mixed An informal approach used to describe the selection, chronicling, and ordering of primary evidence to produce an account of the evidence with commentary and interpretation. It can ‘integrate’ qualitative and quantitative evidence through narrative juxtaposition (discussing diverse forms of evidence side by side). It is less concerned with assessing evidence quality and more focused on gathering relevant information that provides both context and substance to the authors’ overall argument.
Narrative synthesis
(Popay J, 2006) [37]
Qualitative Similar to “Narrative review”, it involves an approach to evidence review but includes a formal analytical process of synthesis to generate new insights or knowledge by seeking to be systematic and transparent. It involves the ‘simple’ juxtaposition of findings from the studies included in the review and some element of integration or interpretation. There are 3 main elements to the process: developing a preliminary synthesis of the findings of included studies; exploring relationships in the data; and assessing the robustness of the synthesis product.
Qualitative cross-case analysis
(Miles & Huberman. 1994; Yin R. 2003) [38, 39]
Mixed Case studies are used to understand complex social phenomena. Research using a case study approach may be based on a single or multiple cases, and can include a mixture of qualitative and quantitative evidence.
Qualitative meta- synthesis
(Jensen & Allen 1996) [40]
Qualitative Meta-synthesis attempts to integrate results from a number of different but inter-related qualitative studies. The technique has an interpretive, rather than aggregating, intent, in contrast to meta-analysis of quantitative studies. Qualitative meta- synthesis defined as theories, grand narratives, generalizations, or interpretive translations produced from the integration or comparison of findings from qualitative studies.
Qualitative systematic review / Qualitative evidence synthesis
(Grant 2009) [41]
Qualitative Method for integrating or comparing findings from qualitative research. The method helps identify themes or constructs that lie in or across individual studies. The resulting accumulated knowledge may lead to the development of a new theory, an overarching “narrative” a wider generalization or “interpretative translation”.
Quantitative case survey
(Yin R and Heald K. 1975; Pelz D. 1981) [42, 43]
Mixed A formal process for systematically coding data from a number of qualitative cases sufficient for quantitative analysis. A set of structured questions is used to extract data from individual case studies, which are then treated as observations within a single dataset. Data are then converted to quantitative form for statistical analysis. It is a way of turning qualitative studies into quantitative data for analysis, allowing an integrated qualitative-quantitative synthesis to be undertaken.
Realist review / synthesis
(Pawson 2005) [10]
Mixed Rooted in philosophy, this is a method used to investigate ‘what works for whom, under what circumstances, and why’. Primary focus is on the causal mechanisms or “theories” that underlie types of interventions or programmes and aims to build explanations across interventions or programmes which share similar underlying “theories of change” as to why they work (or not) for particular groups in particular contexts.
Textual Narrative synthesis
(Lucas, 2007) [44]
Mixed An approach that arranges studies into more homogeneous groups, and useful for synthesizing different types of evidence (quantitative, qualitative, economic, etc). Study characteristics, context, quality and findings are reported according to a standard format, and similarities and differences are compared across studies
Thematic analysis
(Mays, 2005) [45]
Mixed The most common method adopted within ‘Narrative reviews” to produce a relatively rudimentary synthesis of findings across the included studies. It involves identifying prominent or recurring themes in the literature (largely shaped by research questions), and summarizing the findings of different studies under thematic headings using summary tables, which can inform a description of key points.
Thematic synthesis
(Thomas, 2008) [46]
Qualitative This approach combines and adapts approaches from both meta-ethnography and grounded theory. Free codes of findings are organized into ‘descriptive’ themes, which are then further interpreted to yield ‘analytical’ themes (comparable to 3rd order interpretations from meta-ethnography).