What is a systematic review?

A systematic review is a comprehensive and structured synthesis of the available evidence on a specific research question or topic. It follows a predetermined and systematic approach to identifying, evaluating, and summarizing relevant studies, with the aim of providing a robust and unbiased overview of the existing literature. Systematic reviews are commonly conducted in various fields, including medicine, social sciences, education, and more.

A systematic review is commonly characterised by:

• A well-defined research question
• Transparent search terms and database selection
• Exclusion/inclusion criteria with evaluation of search findings
• A research project structure with elements such as Introduction, Method, Result, Discussion

A systematic review is considered secondary research because it uses research by others and does not involve data collection for a new research project.

Key characteristics of a systematic review include:

• Research Question or Objective: Clearly defined research question or objective that the systematic review aims to address.
• Inclusion and Exclusion Criteria: Specific criteria are established for including or excluding studies based on factors such as study design, population, intervention, comparison, outcomes (often referred to as PICO criteria in healthcare).
• Search Strategy: A systematic and exhaustive search of relevant databases, literature repositories, and other sources to identify all potentially eligible studies.
• Study Selection: Rigorous selection process based on the predefined inclusion and exclusion criteria. Multiple researchers often independently screen and select studies to minimize bias.
• Data Extraction: Systematic extraction of key information from selected studies, such as study design, participant characteristics, interventions, outcomes, and results.
• Quality Assessment: Evaluation of the methodological quality of each included study to assess the reliability and validity of their findings.
• Data Synthesis: Integration and synthesis of the results from individual studies using statistical methods if appropriate. This could include meta-analysis, which combines data from multiple studies to derive overall effect estimates.
• Publication Bias Assessment: Evaluation of potential publication bias, where studies with positive or significant results are more likely to be published than those with negative or nonsignificant results.
• Critical Appraisal and Interpretation: Critical appraisal of the overall body of evidence, including strengths and weaknesses of individual studies. Interpretation of findings in the context of the research question.
• Conclusion and Implications: A conclusion that summarizes the evidence and discusses the implications for practice, policy, or future research.

Systematic reviews are considered a gold standard in evidence-based practice because of their methodological rigor and the comprehensive nature of their approach. They provide a valuable resource for researchers, clinicians, policymakers, and other stakeholders seeking a reliable summary of the current state of knowledge on a particular topic.

How is it different from a traditional literature review?

The purpose of systematic review is different from that of a traditional literature review.

Traditional (narrative) literature review	Systematic review
Reviews past research to identify gaps or discrepancies and establish need for new research Provides a foundation that the researcher uses to position their own new research	Uses best available evidence in a body of literature to arrive at a conclusion  Provides a foundation from which the researcher(s) can make recommendations

A systematic review further

• involves a clearly articulated search process and selection criteria of the literature which is closely examined before being included in the review.
• uses a search and selection procedure that is transparent and can be replicated. 

In a traditional literature review, the researcher

• selects and examines studies related to the research topic.
• does not have to make visible the search and selection process and criteria.

Types of systematic reviews

There are four common types of reviews using systematic methods:

• Systematic reviews
• Rapid reviews
• Scoping reviews
• Integrative reviews

Systematic review	Rapid review	Scoping review	Integrative review
Uses best available evidence from a broad range of databases to arrive at a conclusion related to a specific problem or intervention Provides a foundation from which the researcher(s) can make recommendations for theory and practice	Uses best available evidence from a narrow span of data bases to establish what is known about a problem, policy or practice Presents evidence-based summaries to advice stakeholders	Commonly uses a wide range of sources to establish scope, nature and characteristics of a topic or emerging research field, not yet fully reviewed, or which is complex/varied in nature Often provides a preparative review of the nature and extent of research evidence and gaps in knowledge	Uses a variety of methodological, empirical and theoretical studies Provides a detailed understanding of an issue or phenomenon to indicate any gaps in knowledge, develop theory, and inform policy or practice

A common feature of these reviews is the goal of reducing bias in the search and selection of studies.

This bias mainly refers to:

• Availability of resources
• Researcher’s degree of objectivity
• Degree of similarity in type and content of research

A common strategy for reducing bias:

• Extended time to perform a thorough search in published and ‘yet to be published’ articles
• Two or more reviewers following transparent processes of conducting searches and making selections
• Homogeneity of selected research articles

Steps in a systematic review