Systematic Review

What is a Systematic Review?

A systematic review is a rigorous and systematic synthesis of research studies on a specific topic or research question.

The primary goal of a systematic review is to evaluate and summarise all available evidence related to a particular research question in a comprehensive, transparent, and unbiased manner.

Systematic reviews are essential to evidence-based practice because they provide high-quality evidence on the effectiveness of interventions or treatments.

Example of a Systematic Review

A group of students performs a systematic review to assess the effectiveness of mindfulness-based therapy for anxiety disordersand examines all relevant studies published in peer-reviewed journals.

For this purpose, they follow a structured process:

• Define inclusion criteria
• Search multiple databases (e.g., PubMed, PsycINFO)
• Assess study quality

Findings are synthesised to identify overall trends and consistency in outcomes. The conclusion of the review shows that mindfulness-based therapy reduces anxiety symptoms. 

Comparison of Review Types

1. Systematic Review vs Meta-analysis

2. Systematic Review vs Scoping Review

3. Systematic Review vs Literature Review

Key Components of a Systematic Review

The key components are as follows:
 

1. Clearly Defined Question

The review begins with a clearly formulated research question, often framed within the Population, Intervention, Comparison, and Outcome (PICO) framework.
 

2. Protocol Development

Before starting the review, a protocol is developed that outlines the specific methods to be used. This protocol ensures transparency and can be registered on platforms like PROSPERO.
 

3. Search Strategy

A comprehensive, systematic search across multiple databases (e.g., PubMed, Cochrane Library, Embase) should be conducted to identify all relevant studies. Search terms and strategies should be meticulously designed to capture as much relevant literature as possible.
 

4. Study Selection

Using predefined inclusion and exclusion criteria, studies are selected for inclusion in the review. Typically, at least two reviewers carry out this process independently to minimise bias.
 

5. Data Extraction

Relevant data are extracted from the included studies using standardised forms or software. These may consist of details on study design, population, interventions, outcomes, and results.
 

6. Quality Assessment

The quality or risk of bias of the individual studies is assessed using standardised tools. This helps in determining the overall strength of the evidence.
 

7. Data Synthesis

The data from individual studies are synthesised. If studies are sufficiently homogenous in terms of design and outcomes, a meta-analysis may be conducted to pool results statistically.
 

8. Interpretation and Conclusion

The findings are interpreted in the context of the overall evidence, and conclusions are drawn about the research question.
 

9. Reporting

The review is reported in accordance with guidelines such as the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) to ensure transparent and complete reporting.
 

Why Are Systematic Reviews Important?

Here are some reasons why systematic reviews are essential:

• The systematic approach involves a structured, predefined methodology that helps minimise bias. This includes explicit selection criteria, comprehensive literature searches, and standardised evidence appraisal.
• Due to the rigorous methodology, the findings of systematic reviews are often considered more reliable than those of other types of review.
• They can help identify consistency or variability in research findings.
• They play a pivotal role in developing clinical guidelines and consensus statements.
• Beyond healthcare, systematic reviews can influence public policy decisions by providing consolidated evidence on the effectiveness and implications of interventions and policies.

The Steps To Conducting A Systematic Review

Systematic reviews are rigorous, transparent literature reviews designed to answer a specific research question. They provide a comprehensive and exhaustive summary of the current literature relevant to a research question. Here is a comprehensive guide to conducting a systematic review:
 

Step 1. Defining the Research Question

Your research question should be clear and specific. Consider the PICO format:
 

Example

A group of students conducts studies to determine whether drinking milk helps children grow taller.

P: Children aged 7–10

I: Drinking one glass of milk daily

C: Not drinking milk

O: Height growth over six months

Research Question: 

Does drinking milk daily help children grow taller than those who do not drink milk regularly?
 

Step 2. Developing a Protocol

Before you start your search, develop a protocol that outlines your planned methods. This should include:

• The research question
• Eligibility criteria for studies
• Databases and other evaluating sources
• Search terms and strategy
• The process for selecting studies
• Data extraction methods
• Plans for assessing the quality of studies

Step 3. Literature Search

Start research using database platforms such as PubMed, Scopus, Web of Science, and specialised databases relevant to your field.

Use controlled vocabulary and free-text terms. Moreover, all search strategies should be documented to ensure transparency and accountability.

Example

In a literature review, students search online databases such as Google Scholar and PubMed for studies on milk and children’s height.

They use keywords such as “milk effect on growth”, “height growth in children”, and “child growth”.

Every search is carefully recorded so others can follow the same method and check the results later.
 

Step 4. Study Selection

After retrieving studies, eliminate duplicates. Screen titles and abstracts for relevance, followed by full-text reviews. Document reasons for exclusion to ensure transparency and reproducibility.

Example

For study selection, they followed a rigorous process, removing 120 duplicates, screening 300 titles/abstracts, and excluding 250 for incorrect age or outcome.

Moreover, retrieving 50 full texts, excluding 40 with reasons (no comparator, short follow-up), and finally including 10 studies.
 

Step 5. Data Extraction

For each included study, extract relevant data using standardised forms. This may include:

• Study design
• Population characteristics
• Interventions or exposures
• Outcome measures
• Findings

Example

For precise, practical data extraction, they focused on the following factors.

• Study ID
• Design
• Sample size
• Age range
• Intervention dose
• Comparator
• Follow-up duration
• Height measurement method
• Mean height change
• Standard deviations (SDs)
• Funding source
• The authors’ main conclusions were recorded in a spreadsheet.

Step 6. Assessment of Study Quality

Evaluate the quality or risk of bias in each study. Tools such as the Cochrane Risk of Bias Tool or the Newcastle-Ottawa Scale can be employed, depending on the study design.
 

Example

To assess study quality, students used the Cochrane RoB tool for RCTs, rating randomisation, allocation concealment, blinding, incomplete outcome data, and selective reporting.

​In the process of assessing study quality, they flagged three trials as high risk due to insufficient blinding and missing data.
 

Step 7. Data Synthesis

Combine and summarise the findings. If the studies are sufficiently homogeneous, consider a meta-analysis.

This statistical method combines findings from multiple studies to derive an overall effect size. If a meta-analysis is not feasible, provide a narrative synthesis.
 

Example

For data synthesis, they assessed heterogeneity (I² = 45%).

They performed a random-effects meta-analysis of mean height change; the pooled effect was +0.8 cm (95% CI 0.2–1.4).

This was supplemented with a narrative summary for cohort studies.
 

Step 8. Interpretation and Report Writing

Discuss the findings in context, consider the strengths and limitations of the included studies, and draw conclusions based on these findings. Remember to:

• Highlight the implications of the findings.
• Suggest areas for future research.
• Follow reporting guidelines, such as the PRISMA guidelines for systematic reviews.

Example

After synthesising all the data, they interpreted the results, wrote the reports, and reported a modest height increase with daily milk, noting a small effect size, sources of heterogeneity, and risks of bias.

The reports recommended cautious interpretation, implications for dietary guidance, and gaps for future high-quality trials.
 

Step 9. Peer Review and Publication

Submit the systematic review to a relevant journal for peer review and consideration. Expert feedback can significantly enhance the quality and validity of your findings.
 

Example

Last but not least, they shared the first draft with supervisors and peers, incorporated their feedback on methods and clarity, and revised figures and the bias discussion. 

After finalising the first systematic review, they submitted it to a paediatric nutrition journal for peer review.​
 

Step 10. Stay Updated

• Systematic reviews can become outdated. Set up alerts to notify you of new studies that meet your inclusion criteria so you can keep the review current.
• Consider updating the review after a certain number of new relevant studies are published.

Challenges In Conducting Systematic Reviews

Systematic reviews are essential for summarising the best available evidence on a particular topic, guiding clinical decisions, and identifying gaps in current research.

• Extracting data from studies can be labour-intensive and sometimes challenging if the data are presented in a non-standardised way or if essential details are missing.
• Differences across studies in their populations, interventions, outcomes, and methodologies can make it difficult to combine data.
• Deciding how to assess the quality and risk of bias in the included studies can be challenging.
• Systematic reviews require significant time, expertise, and effort to manage sources, making them expensive and sometimes prohibitive for researchers.
• This includes non-peer-reviewed articles, reports, theses, and other similar works.

Tips For A Successful Systematic Review

Here are some tips to ensure the success of a systematic review:

• Use the PICO framework to clearly define your research question.
• Register your review on PROSPERO to ensure transparency and avoid duplication.
• Include grey literature and scan reference lists for additional studies.
• Use structured data extraction forms and record reasons for exclusions.
• Assess each study’s quality to ensure reliability and validity.
• Check for heterogeneity before combining study results.
• Choose fixed-effect or random-effects models appropriately.
• Keep refining your research strategy as new evidence emerges.

Frequently Asked Questions