Literature Searching and Reviews

Selected types of reviews characterized after methods.

Type and description

Searches

Quality Assessment

Synthesis

Analysis

Critical review
Comprehensive examination of the literature and critical evaluation of quality. Not just description but adding analysis or theoretical innovation. Result often a hypothesis or model.

Identification of the most important works on the topic.

No formal quality assessment, but evaluation of the quality of the work's contribution to the field.

Typically narrative, conceptual, or chronological.

Identifying theoretical contributions by incorporating existing theory or deriving new theory.

Literature review or
State of the Art

Generic term for reviewing new or recent literature. Covers very varied degrees of comprehensiveness. May include research findings.

May contain comprehensive searches

May include quality assessment

Typically, narrative

Chronological, theoretical, thematic, etc.

Mapping review/
Systematic Map

Maps and categorizes literature and identifies gaps in research.

The comprehensiveness of the search is limited by time and scope.

No formal quality assessment

Typically graphical or tabular

Describes the quantity and quality of literature e.g., by study design or other key elements.

Meta-Analysis

Statistical method that combines results of quantitative studies for more accurate estimation of effect.

Provides a comprehensive and exhaustive search. Can use funnel plot to visualize the completeness of the search.

The quality assessment forms the basis for inclusion/exclusion and/or sensitivity analysis.

Graphical or tabular with narrative comments

Numerical analysis of endpoints. Heterogeneity in the studies is expected to be absent.

Mixed studies review/
Mixed methods reviews

Any combination of methods, an essential component of which is a literature review (usually systematic). This means a combination of e.g., qualitative, and quantitative research and outcome and process studies.

Requires either extremely sensitive search (identifying all relevant studies) or separate qualitative and quantitative search strategies.

Requires either generic quality assessment methodology or separate processes with corresponding checklists.

Both components are typically described narratively and in tables. Also, graphical methods (combination of qualitative and quantitative studies).

Can describe both types of studies and examine correlations between characteristics. Or use gap analysis to identify aspects in one type of study that are missing in another type of study.

Narrative reviews
Subjectively selected body of literature with the aim of presenting the current knowledge in each field.

Rarely in-depth description of search methods. Not necessarily systematic search.

No formal quality assessment

Narrative synthesis

Narrative analysis

Overview

Generic term: Summary of (medical) literature that reviews the literature and describes its characteristics.

Can be an extensive literature search, depending on how systematic the overview is.

May apply formal quality assessment, depending on how systematic the overview is.

The synthesis depends on how systematic the approach is. Most often narrative but can include tables.

Chronological, theoretical, thematic, etc.

Qualitative systematic re-view/qualitative evidence synthesis

Methods for synthesizing or comparing qualitative studies. Attempting to form themes or theories based on conceptual elements across individual qualitative studies.

Can apply selective or purposive sampling of studies.

Quality assessment on overall criteria: Credibility, transferability, reliability, and verifiability.

Qualitative, narrative synthesis

Thematic analyses may include theoretical models.

Rapid review

Assess existing knowledge of a policy or practice topic. Apply systematic review methods to search for and assess the quality of existing research. Often a short deadline.

The execution of the literature search depends on any time constraints on the project.

Time-limited formal quality assessment

Typically narrative and tabular

Description of the amount of literature and the overall quality and direction of the evidence.

Realist reviews

Summarizes knowledge about the mechanisms behind why and how complex social interventions work (or do not work) in specific situations. Includes several types of studies and a large amount of gray literature.

Multiple search methods for purposive sampling of literature for inclusion. An iterative search process only stops when no literature is identified that contributes to new understanding of the intervention.

Quality assessment is based on whether the method fits the research question (fitness-for-purpose). Based on assessment of both relevance and methodogical consistency.

Narrative, explanatory analysis and starting point in program theory. Combines (social) theory with empirical evidence to summarize and further develop program theory.

Provides recommendations for policymakers around the intervention, especially regarding contextual factors

Scoping review/Evidence Map

Initial assessment of potential volume of scientific literature. Goal: identifies the type and extent of evidence (incl. typical ongoing research). Overview of a wide field. 

Conducting the search depends on time and scope limitations of the review. May include ongoing research.

No formal quality assessment

Tabular format with narrative comments, visual presentation of the evidence.

Characterizes the quantity and quality of the literature, e.g., by study design or other key elements. Can point out the need for new reviews.

Systematic review

Refers to a systematic search, quality assessment and synthesis of evidence. Often based on guidelines for the preparation of systematic reviews.

Provides a comprehensive and exhaustive search.

PRISMA provides a good overview of the search.

The quality assessment can determine inclusion/exclusion.

Typically narrative, supplemented with table.

Summarizes existing knowledge with recommendations for practice. Points out knowledge gaps and uncertainties in the results and recommendations for future research.

Systematic search and review
Combine the strengths of the critical review with a comprehensive search process. Typically addresses broad questions to provide a 'best evidence synthesis'.

Provides comprehensive and exhaustive search.

May include quality assessment

Minimal narrative and schematic summarization of studies.

Summarizes existing knowledge, limitations, and recommendations for practice.

Systematized review
Attempts to include elements of the systematic review process but stops short of an actual systematic review. Can typically be a postgraduate assignment.

May include a comprehensive search.

May include quality assessment

Typically narrative, supplemented with table.

Summarizes existing knowledge, uncertainty of results.

Methodogical limitations.

Umbrella review
Specifically refers to reviews that synthesize evidence from multiple reviews into one accessible and usable document. Focuses on broad problem with competing interventions.

Identifying reviews but not searching for primary studies.

Quality assessment of primary studies in the included reviews or of entire reviews.

Graphically and in table form with narrative comments.

Summarizes existing knowledge with recommendations for practice. Identifies knowledge gaps and provides recommendations for future research.