Synthesising Evidence and Identifying Research Gaps

Overview

Evidence synthesis combines findings across sources to answer a question. It does more than list one article after another. A synthesis identifies patterns, differences, strengths, limitations and explanations across the evidence base. Its structure should follow the review purpose and the intended research or clinical decision.

A research gap is not simply a topic with few publications. It is an uncertainty that remains after considering the relevance, methods, consistency and applicability of existing evidence. The choice between mapping and answer-focused synthesis should remain aligned with the purpose of the review [1]. The gap should be consequential and capable of being addressed by an appropriate new study.

Why this matters

An article-by-article review transfers the burden of synthesis to the reader. It can give equal weight to weak and strong evidence, hide repeated use of the same dataset and make contradictions appear greater than they are. A structured synthesis shows how studies relate to one another.

Research gaps are also easy to overstate. “No study has been conducted in our hospital” does not establish a gap unless local context plausibly changes the answer or a local decision requires local data. Conversely, many publications do not mean that the question is settled if measurements are poor, important populations are absent or findings cannot be applied.

Preparing for synthesis

Before writing, construct evidence tables that organise studies by fields relevant to the question:

• population and setting;
• design and data source;
• definitions and measurement;
• intervention or exposure;
• follow-up and attrition;
• principal findings;
• methodological limitations; and
• contextual factors.

Check whether multiple reports use the same dataset. Do not treat repeated analyses as independent replication.

Forms of synthesis

Descriptive mapping

Summarise the distribution of evidence by year, country, setting, design, population, outcome or measurement approach. This is central to scoping reviews and useful before deeper synthesis.

Narrative synthesis

Narrative synthesis uses structured text and tables to explain patterns across quantitative or mixed evidence when statistical pooling is not performed or is only one component. Guidance recommends developing a theory of how an intervention works, constructing a preliminary synthesis, exploring relationships and assessing the robustness of the synthesis [2].

Thematic synthesis

Qualitative findings can be coded and organised into descriptive and analytical themes. The process should preserve context and distinguish participant data, study-author interpretation and review-author synthesis [3].

Quantitative synthesis

Meta-analysis may combine sufficiently compatible numerical estimates. When meta-analysis is not undertaken, structured reporting guidance such as SWiM can improve transparency of synthesis methods and findings [4]. Detailed statistical methods belong in the Biostatistics section. Methodologically, the review must first establish that the studies address a coherent estimand and that differences in design, population and measurement do not make pooling misleading.

Framework or model development

An integrative or conceptual synthesis may arrange evidence into a causal, implementation or process framework. The model should distinguish evidence-supported relationships from proposed hypotheses.

Synthesis matrix for postpartum follow-up

illustrative

Table 1. Organising evidence by explanatory dimension

Dimension	Pattern across studies	Possible explanation	Remaining uncertainty
Referral pathway	Higher completion where appointments were arranged before discharge	Reduced coordination burden	Which component of the pathway is essential?
Test definition	Studies used different tests and time windows	Guideline and service variation	How should completion be standardised locally?
Patient barriers	Travel, childcare and low perceived risk recur	Follow-up competes with postpartum demands	Which barriers are modifiable in the local setting?
Reminder interventions	Effects vary	Reminders may fail when access barriers remain	Is navigation required in addition to reminders?

Note: Patterns are illustrative rather than published estimates.

Explaining inconsistent findings

Inconsistency should be investigated, not merely labelled. Differences may arise from:

Population

Risk, age, socioeconomic position, comorbidity, parity and cultural context may differ.

Setting and care pathway

Referral systems, test availability, cost, distance and integration between maternity and primary care can modify participation and intervention effects.

Definitions and measurement

Studies may define completion using different tests, time windows, data sources or denominators. Apparent disagreement may disappear when definitions are aligned.

Design and selection

A voluntary survey, retrospective record study and population cohort may include different participants. Loss to follow-up and missing linkage can alter estimates.

Intervention content and implementation

Two “reminder” interventions may differ in channel, timing, frequency, language, navigation support and fidelity.

Analysis and reporting

Different adjustment sets, effect measures, subgroup definitions and selective outcome reporting can produce different conclusions.

Assessing the robustness of the synthesis

Ask whether the main conclusions change when considering:

• studies at high risk of bias;
• small or imprecise studies;
• indirect populations or settings;
• inconsistent definitions;
• unpublished or grey evidence;
• missing outcomes;
• duplicated datasets; and
• reviewer interpretation.

A synthesis should communicate uncertainty. Statements such as “all studies show” or “the evidence proves” are rarely warranted.

Identifying research gaps

A useful gap can occur in several dimensions.

Evidence gap

The question has not been studied or evidence is sparse.

Population gap

Important groups are absent or combined in ways that prevent relevant conclusions.

Context gap

Evidence arises from settings with different systems, resources, epidemiology or social conditions.

Intervention or exposure gap

A promising strategy, exposure range or implementation component has not been evaluated.

Outcome or measurement gap

Studies use surrogate, inconsistent, invalid or short-term outcomes.

Design gap

Existing designs cannot address temporality, causal inference, experience, long-term follow-up or real-world implementation.

Methodological-quality gap

Studies exist but are too biased, imprecise or poorly reported to support a decision.

Synthesis gap

Primary studies exist, but they have not been brought together in a current, rigorous review.

Implementation gap

Effectiveness is established, but adoption, reach, fidelity, cost, acceptability or sustainability remains uncertain.

From literature finding to researchable gap

illustrative

Table 2. Weak and stronger gap statements

Weak statement	Stronger statement
No study has been done in our hospital.	Existing studies arise from integrated health systems, whereas local follow-up requires transfer between unlinked maternity and primary-care records; the stages of local loss are unknown.
Results are inconsistent.	Reminder effects vary according to whether appointment booking and navigation were included, but intervention components are incompletely reported.
More research is needed.	Prospective evaluation is needed to determine whether a discharge-booking plus navigation pathway improves test completion and remains feasible in district clinics.

Prioritising gaps

Not every gap deserves a new study. Prioritise by:

• importance to patients, services or policy;
• likelihood that the answer changes a decision;
• ethical acceptability;
• feasibility;
• availability of an appropriate design and measurement;
• duplication with ongoing studies; and
• stakeholder priorities.

DECISION GUIDE

Does the gap justify a new study?

1. Is the uncertainty real after a credible search and synthesis?
2. Is it consequential for health, practice, policy or theory?
3. Can a new study reduce the uncertainty using an appropriate design?
4. Is the required population or data accessible ethically?
5. Would an update, reanalysis, audit or implementation project be more suitable?
6. Are ongoing studies already likely to answer the question?

Linking synthesis to the proposed study

The literature review should explain how the synthesis informed:

• the research question and objectives;
• the conceptual framework;
• population and setting;
• design;
• variables or qualitative domains;
• operational definitions;
• measurement tools;
• bias-control strategies;
• follow-up period;
• feasibility assumptions; and
• analysis interface.

The proposed study should respond to the gap, not merely occur in the same topic area.

PROTOCOL CHECKLIST

Write the synthesis before the study justification

• Group evidence by concepts or methodological dimensions.
• Compare definitions, populations, designs and settings.
• Identify consistent findings and credible disagreements.
• Evaluate trustworthiness and applicability.
• State what remains unknown and why.
• Check ongoing studies and recent reviews.
• Prioritise one consequential gap.
• Show how each major design decision responds to the synthesis.
• Avoid claims that cannot be supported by the search scope.

Research-gap frameworksMMed extension

Frameworks such as population, intervention, comparison, outcome and setting can classify gaps, while evidence-gap maps display the distribution of studies. These tools organise uncertainty but do not decide importance. Priority setting still requires judgement about consequences, feasibility, equity and stakeholder needs [5]. JBI guidance similarly treats gap identification as a product of explicit evidence mapping, appraisal and synthesis rather than publication counting alone [6].

Worked example

WORKED EXAMPLE

Synthesising the postpartum follow-up evidence

Descriptive pattern: Completion estimates vary widely, but studies use different tests, follow-up windows and data-linkage methods.

Barrier synthesis: Travel, competing childcare, low perceived risk and unclear referral recur across settings. Provider knowledge alone does not explain non-completion.

Intervention synthesis: Simple reminders appear less effective where women must independently arrange testing. Multicomponent pathways that book appointments or provide navigation may perform better, but intervention descriptions and implementation outcomes are incomplete.

Context assessment: Most intervention evidence comes from integrated systems with linked records. The local service transfers women from hospital maternity care to separate primary-care clinics.

Research gap: The local points of loss, modifiable barriers and feasibility of a coordinated booking-and-navigation pathway are unknown.

Study response: A mixed-methods feasibility study could map the pathway, estimate recruitment and completion, assess acceptability and refine the intervention before a definitive effectiveness evaluation.

Appraising a synthesis and gap statement

APPRAISAL CHECKLIST

Synthesis and gap appraisal questions

• Is the writing organised by concepts rather than one paragraph per article?
• Are study design, measurement and context considered when comparing findings?
• Are stronger and weaker studies distinguished?
• Are inconsistent findings explored rather than averaged verbally?
• Are multiple reports or repeated datasets recognised?
• Is the proposed gap supported by the search and synthesis?
• Is the gap consequential and researchable?
• Does the proposed study directly address the stated uncertainty?
• Are novelty and causal claims appropriately limited?

Suggested protocol or reporting wording

Common mistakes

Knowledge check

What distinguishes synthesis from an article-by-article summary?

Answer and reasoning

Synthesis compares and integrates studies around the review question. It identifies patterns, differences, explanations and uncertainty across studies rather than reporting each paper separately.

Several studies report different completion rates. Does this establish true disagreement?

Answer and reasoning

Not necessarily. The studies may use different test definitions, follow-up windows, data sources, populations or denominators. These methodological and contextual differences should be examined before concluding that findings conflict.

Why is ‘no study has been done in our hospital’ an incomplete gap statement?

Answer and reasoning

Location alone does not show that existing evidence is inapplicable or that a local study would change a decision. The researcher should explain which contextual feature creates uncertainty and what information the local study would provide.

When might an implementation gap remain despite evidence of effectiveness?

Answer and reasoning

An intervention may work under study conditions while its acceptability, reach, fidelity, cost, equity or sustainability in routine services remains unknown. An implementation-focused study may then be more useful than another efficacy trial.

Key take-home messages

SUMMARY

Key take-home messages

• Synthesis organises evidence around questions and explanatory dimensions, not individual papers.
• Differences in population, design, measurement and context can explain apparent inconsistency.
• A research gap is a consequential unresolved uncertainty, not merely a low publication count.
• Prioritise gaps that can change understanding or decisions and can be addressed ethically and feasibly.
• The proposed study must respond directly to the gap identified by the synthesis.

References

1. Munn Z, Peters MDJ, Stern C, Tufanaru C, McArthur A, Aromataris E. Systematic review or scoping review? Guidance for authors when choosing between a systematic or scoping review approach. BMC Med Res Methodol. 2018;18:143. https://doi.org/10.1186/s12874-018-0611-x
2. Popay J, Roberts H, Sowden A, Petticrew M, Arai L, Rodgers M, et al. Guidance on the conduct of narrative synthesis in systematic reviews. Lancaster: Lancaster University; 2006.
3. Thomas J, Harden A. Methods for the thematic synthesis of qualitative research in systematic reviews. BMC Med Res Methodol. 2008;8:45. https://doi.org/10.1186/1471-2288-8-45
4. Campbell M, McKenzie JE, Sowden A, Katikireddi SV, Brennan SE, Ellis S, et al. Synthesis without meta-analysis (SWiM) in systematic reviews: reporting guideline. BMJ. 2020;368:l6890. https://doi.org/10.1136/bmj.l6890
5. Robinson KA, Saldanha IJ, McKoy NA. Development of a framework to identify research gaps from systematic reviews. J Clin Epidemiol. 2011;64(12):1325–1330. https://doi.org/10.1016/j.jclinepi.2011.06.009
6. Aromataris E, Lockwood C, Porritt K, Pilla B, Jordan Z, editors. JBI Manual for Evidence Synthesis. Adelaide: JBI; 2024. https://synthesismanual.jbi.global