does a systematic review have a hypothesis

Have a language expert improve your writing

Run a free plagiarism check in 10 minutes, generate accurate citations for free.

• Knowledge Base

Methodology

• Systematic Review | Definition, Example, & Guide

Systematic Review | Definition, Example & Guide

Published on June 15, 2022 by Shaun Turney . Revised on November 20, 2023.

A systematic review is a type of review that uses repeatable methods to find, select, and synthesize all available evidence. It answers a clearly formulated research question and explicitly states the methods used to arrive at the answer.

They answered the question “What is the effectiveness of probiotics in reducing eczema symptoms and improving quality of life in patients with eczema?”

In this context, a probiotic is a health product that contains live microorganisms and is taken by mouth. Eczema is a common skin condition that causes red, itchy skin.

Table of contents

What is a systematic review, systematic review vs. meta-analysis, systematic review vs. literature review, systematic review vs. scoping review, when to conduct a systematic review, pros and cons of systematic reviews, step-by-step example of a systematic review, other interesting articles, frequently asked questions about systematic reviews.

A review is an overview of the research that’s already been completed on a topic.

What makes a systematic review different from other types of reviews is that the research methods are designed to reduce bias . The methods are repeatable, and the approach is formal and systematic:

• Formulate a research question
• Develop a protocol
• Search for all relevant studies
• Apply the selection criteria
• Extract the data
• Synthesize the data
• Write and publish a report

Although multiple sets of guidelines exist, the Cochrane Handbook for Systematic Reviews is among the most widely used. It provides detailed guidelines on how to complete each step of the systematic review process.

Systematic reviews are most commonly used in medical and public health research, but they can also be found in other disciplines.

Systematic reviews typically answer their research question by synthesizing all available evidence and evaluating the quality of the evidence. Synthesizing means bringing together different information to tell a single, cohesive story. The synthesis can be narrative ( qualitative ), quantitative , or both.

Here's why students love Scribbr's proofreading services

Discover proofreading & editing

Systematic reviews often quantitatively synthesize the evidence using a meta-analysis . A meta-analysis is a statistical analysis, not a type of review.

A meta-analysis is a technique to synthesize results from multiple studies. It’s a statistical analysis that combines the results of two or more studies, usually to estimate an effect size .

A literature review is a type of review that uses a less systematic and formal approach than a systematic review. Typically, an expert in a topic will qualitatively summarize and evaluate previous work, without using a formal, explicit method.

Although literature reviews are often less time-consuming and can be insightful or helpful, they have a higher risk of bias and are less transparent than systematic reviews.

Similar to a systematic review, a scoping review is a type of review that tries to minimize bias by using transparent and repeatable methods.

However, a scoping review isn’t a type of systematic review. The most important difference is the goal: rather than answering a specific question, a scoping review explores a topic. The researcher tries to identify the main concepts, theories, and evidence, as well as gaps in the current research.

Sometimes scoping reviews are an exploratory preparation step for a systematic review, and sometimes they are a standalone project.

Receive feedback on language, structure, and formatting

Professional editors proofread and edit your paper by focusing on:

• Academic style
• Vague sentences
• Style consistency

See an example

A systematic review is a good choice of review if you want to answer a question about the effectiveness of an intervention , such as a medical treatment.

To conduct a systematic review, you’ll need the following:

• A precise question , usually about the effectiveness of an intervention. The question needs to be about a topic that’s previously been studied by multiple researchers. If there’s no previous research, there’s nothing to review.
• If you’re doing a systematic review on your own (e.g., for a research paper or thesis ), you should take appropriate measures to ensure the validity and reliability of your research.
• Access to databases and journal archives. Often, your educational institution provides you with access.
• Time. A professional systematic review is a time-consuming process: it will take the lead author about six months of full-time work. If you’re a student, you should narrow the scope of your systematic review and stick to a tight schedule.
• Bibliographic, word-processing, spreadsheet, and statistical software . For example, you could use EndNote, Microsoft Word, Excel, and SPSS.

A systematic review has many pros .

• They minimize research bias by considering all available evidence and evaluating each study for bias.
• Their methods are transparent , so they can be scrutinized by others.
• They’re thorough : they summarize all available evidence.
• They can be replicated and updated by others.

Systematic reviews also have a few cons .

• They’re time-consuming .
• They’re narrow in scope : they only answer the precise research question.

The 7 steps for conducting a systematic review are explained with an example.

Step 1: Formulate a research question

Formulating the research question is probably the most important step of a systematic review. A clear research question will:

• Allow you to more effectively communicate your research to other researchers and practitioners
• Guide your decisions as you plan and conduct your systematic review

A good research question for a systematic review has four components, which you can remember with the acronym PICO :

• Population(s) or problem(s)
• Intervention(s)
• Comparison(s)

You can rearrange these four components to write your research question:

• What is the effectiveness of I versus C for O in P ?

Sometimes, you may want to include a fifth component, the type of study design . In this case, the acronym is PICOT .

• Type of study design(s)
• The population of patients with eczema
• The intervention of probiotics
• In comparison to no treatment, placebo , or non-probiotic treatment
• The outcome of changes in participant-, parent-, and doctor-rated symptoms of eczema and quality of life
• Randomized control trials, a type of study design

Their research question was:

• What is the effectiveness of probiotics versus no treatment, a placebo, or a non-probiotic treatment for reducing eczema symptoms and improving quality of life in patients with eczema?

Step 2: Develop a protocol

A protocol is a document that contains your research plan for the systematic review. This is an important step because having a plan allows you to work more efficiently and reduces bias.

Your protocol should include the following components:

• Background information : Provide the context of the research question, including why it’s important.
• Research objective (s) : Rephrase your research question as an objective.
• Selection criteria: State how you’ll decide which studies to include or exclude from your review.
• Search strategy: Discuss your plan for finding studies.
• Analysis: Explain what information you’ll collect from the studies and how you’ll synthesize the data.

If you’re a professional seeking to publish your review, it’s a good idea to bring together an advisory committee . This is a group of about six people who have experience in the topic you’re researching. They can help you make decisions about your protocol.

It’s highly recommended to register your protocol. Registering your protocol means submitting it to a database such as PROSPERO or ClinicalTrials.gov .

Step 3: Search for all relevant studies

Searching for relevant studies is the most time-consuming step of a systematic review.

To reduce bias, it’s important to search for relevant studies very thoroughly. Your strategy will depend on your field and your research question, but sources generally fall into these four categories:

• Databases: Search multiple databases of peer-reviewed literature, such as PubMed or Scopus . Think carefully about how to phrase your search terms and include multiple synonyms of each word. Use Boolean operators if relevant.
• Handsearching: In addition to searching the primary sources using databases, you’ll also need to search manually. One strategy is to scan relevant journals or conference proceedings. Another strategy is to scan the reference lists of relevant studies.
• Gray literature: Gray literature includes documents produced by governments, universities, and other institutions that aren’t published by traditional publishers. Graduate student theses are an important type of gray literature, which you can search using the Networked Digital Library of Theses and Dissertations (NDLTD) . In medicine, clinical trial registries are another important type of gray literature.
• Experts: Contact experts in the field to ask if they have unpublished studies that should be included in your review.

At this stage of your review, you won’t read the articles yet. Simply save any potentially relevant citations using bibliographic software, such as Scribbr’s APA or MLA Generator .

• Databases: EMBASE, PsycINFO, AMED, LILACS, and ISI Web of Science
• Handsearch: Conference proceedings and reference lists of articles
• Gray literature: The Cochrane Library, the metaRegister of Controlled Trials, and the Ongoing Skin Trials Register
• Experts: Authors of unpublished registered trials, pharmaceutical companies, and manufacturers of probiotics

Step 4: Apply the selection criteria

Applying the selection criteria is a three-person job. Two of you will independently read the studies and decide which to include in your review based on the selection criteria you established in your protocol . The third person’s job is to break any ties.

To increase inter-rater reliability , ensure that everyone thoroughly understands the selection criteria before you begin.

If you’re writing a systematic review as a student for an assignment, you might not have a team. In this case, you’ll have to apply the selection criteria on your own; you can mention this as a limitation in your paper’s discussion.

You should apply the selection criteria in two phases:

• Based on the titles and abstracts : Decide whether each article potentially meets the selection criteria based on the information provided in the abstracts.
• Based on the full texts: Download the articles that weren’t excluded during the first phase. If an article isn’t available online or through your library, you may need to contact the authors to ask for a copy. Read the articles and decide which articles meet the selection criteria.

It’s very important to keep a meticulous record of why you included or excluded each article. When the selection process is complete, you can summarize what you did using a PRISMA flow diagram .

Next, Boyle and colleagues found the full texts for each of the remaining studies. Boyle and Tang read through the articles to decide if any more studies needed to be excluded based on the selection criteria.

When Boyle and Tang disagreed about whether a study should be excluded, they discussed it with Varigos until the three researchers came to an agreement.

Step 5: Extract the data

Extracting the data means collecting information from the selected studies in a systematic way. There are two types of information you need to collect from each study:

• Information about the study’s methods and results . The exact information will depend on your research question, but it might include the year, study design , sample size, context, research findings , and conclusions. If any data are missing, you’ll need to contact the study’s authors.
• Your judgment of the quality of the evidence, including risk of bias .

You should collect this information using forms. You can find sample forms in The Registry of Methods and Tools for Evidence-Informed Decision Making and the Grading of Recommendations, Assessment, Development and Evaluations Working Group .

Extracting the data is also a three-person job. Two people should do this step independently, and the third person will resolve any disagreements.

They also collected data about possible sources of bias, such as how the study participants were randomized into the control and treatment groups.

Step 6: Synthesize the data

Synthesizing the data means bringing together the information you collected into a single, cohesive story. There are two main approaches to synthesizing the data:

• Narrative ( qualitative ): Summarize the information in words. You’ll need to discuss the studies and assess their overall quality.
• Quantitative : Use statistical methods to summarize and compare data from different studies. The most common quantitative approach is a meta-analysis , which allows you to combine results from multiple studies into a summary result.

Generally, you should use both approaches together whenever possible. If you don’t have enough data, or the data from different studies aren’t comparable, then you can take just a narrative approach. However, you should justify why a quantitative approach wasn’t possible.

Boyle and colleagues also divided the studies into subgroups, such as studies about babies, children, and adults, and analyzed the effect sizes within each group.

Step 7: Write and publish a report

The purpose of writing a systematic review article is to share the answer to your research question and explain how you arrived at this answer.

Your article should include the following sections:

• Abstract : A summary of the review
• Introduction : Including the rationale and objectives
• Methods : Including the selection criteria, search method, data extraction method, and synthesis method
• Results : Including results of the search and selection process, study characteristics, risk of bias in the studies, and synthesis results
• Discussion : Including interpretation of the results and limitations of the review
• Conclusion : The answer to your research question and implications for practice, policy, or research

To verify that your report includes everything it needs, you can use the PRISMA checklist .

Once your report is written, you can publish it in a systematic review database, such as the Cochrane Database of Systematic Reviews , and/or in a peer-reviewed journal.

In their report, Boyle and colleagues concluded that probiotics cannot be recommended for reducing eczema symptoms or improving quality of life in patients with eczema. Note Generative AI tools like ChatGPT can be useful at various stages of the writing and research process and can help you to write your systematic review. However, we strongly advise against trying to pass AI-generated text off as your own work.

If you want to know more about statistics , methodology , or research bias , make sure to check out some of our other articles with explanations and examples.

• Student’s  t -distribution
• Normal distribution
• Null and Alternative Hypotheses
• Chi square tests
• Confidence interval
• Quartiles & Quantiles
• Cluster sampling
• Stratified sampling
• Data cleansing
• Reproducibility vs Replicability
• Peer review
• Prospective cohort study

Research bias

• Implicit bias
• Cognitive bias
• Placebo effect
• Hawthorne effect
• Hindsight bias
• Affect heuristic
• Social desirability bias

A literature review is a survey of scholarly sources (such as books, journal articles, and theses) related to a specific topic or research question .

It is often written as part of a thesis, dissertation , or research paper , in order to situate your work in relation to existing knowledge.

A literature review is a survey of credible sources on a topic, often used in dissertations , theses, and research papers . Literature reviews give an overview of knowledge on a subject, helping you identify relevant theories and methods, as well as gaps in existing research. Literature reviews are set up similarly to other  academic texts , with an introduction , a main body, and a conclusion .

An  annotated bibliography is a list of  source references that has a short description (called an annotation ) for each of the sources. It is often assigned as part of the research process for a  paper .  

A systematic review is secondary research because it uses existing research. You don’t collect new data yourself.

Cite this Scribbr article

If you want to cite this source, you can copy and paste the citation or click the “Cite this Scribbr article” button to automatically add the citation to our free Citation Generator.

Turney, S. (2023, November 20). Systematic Review | Definition, Example & Guide. Scribbr. Retrieved August 29, 2024, from https://www.scribbr.com/methodology/systematic-review/

Is this article helpful?

Shaun Turney

Other students also liked, how to write a literature review | guide, examples, & templates, how to write a research proposal | examples & templates, what is critical thinking | definition & examples, "i thought ai proofreading was useless but..".

I've been using Scribbr for years now and I know it's a service that won't disappoint. It does a good job spotting mistakes”

Systematic Reviews and Meta Analysis

• Getting Started
• Guides and Standards
• Review Protocols
• Databases and Sources
• Randomized Controlled Trials
• Controlled Clinical Trials
• Observational Designs
• Tests of Diagnostic Accuracy
• Software and Tools
• Where do I get all those articles?
• Collaborations
• EPI 233/528
• Countway Mediated Search
• Risk of Bias (RoB)

Systematic review Q & A

What is a systematic review.

A systematic review is guided filtering and synthesis of all available evidence addressing a specific, focused research question, generally about a specific intervention or exposure. The use of standardized, systematic methods and pre-selected eligibility criteria reduce the risk of bias in identifying, selecting and analyzing relevant studies. A well-designed systematic review includes clear objectives, pre-selected criteria for identifying eligible studies, an explicit methodology, a thorough and reproducible search of the literature, an assessment of the validity or risk of bias of each included study, and a systematic synthesis, analysis and presentation of the findings of the included studies. A systematic review may include a meta-analysis.

For details about carrying out systematic reviews, see the Guides and Standards section of this guide.

Is my research topic appropriate for systematic review methods?

A systematic review is best deployed to test a specific hypothesis about a healthcare or public health intervention or exposure. By focusing on a single intervention or a few specific interventions for a particular condition, the investigator can ensure a manageable results set. Moreover, examining a single or small set of related interventions, exposures, or outcomes, will simplify the assessment of studies and the synthesis of the findings.

Systematic reviews are poor tools for hypothesis generation: for instance, to determine what interventions have been used to increase the awareness and acceptability of a vaccine or to investigate the ways that predictive analytics have been used in health care management. In the first case, we don't know what interventions to search for and so have to screen all the articles about awareness and acceptability. In the second, there is no agreed on set of methods that make up predictive analytics, and health care management is far too broad. The search will necessarily be incomplete, vague and very large all at the same time. In most cases, reviews without clearly and exactly specified populations, interventions, exposures, and outcomes will produce results sets that quickly outstrip the resources of a small team and offer no consistent way to assess and synthesize findings from the studies that are identified.

If not a systematic review, then what?

You might consider performing a scoping review . This framework allows iterative searching over a reduced number of data sources and no requirement to assess individual studies for risk of bias. The framework includes built-in mechanisms to adjust the analysis as the work progresses and more is learned about the topic. A scoping review won't help you limit the number of records you'll need to screen (broad questions lead to large results sets) but may give you means of dealing with a large set of results.

This tool can help you decide what kind of review is right for your question.

Can my student complete a systematic review during her summer project?

Probably not. Systematic reviews are a lot of work. Including creating the protocol, building and running a quality search, collecting all the papers, evaluating the studies that meet the inclusion criteria and extracting and analyzing the summary data, a well done review can require dozens to hundreds of hours of work that can span several months. Moreover, a systematic review requires subject expertise, statistical support and a librarian to help design and run the search. Be aware that librarians sometimes have queues for their search time. It may take several weeks to complete and run a search. Moreover, all guidelines for carrying out systematic reviews recommend that at least two subject experts screen the studies identified in the search. The first round of screening can consume 1 hour per screener for every 100-200 records. A systematic review is a labor-intensive team effort.

How can I know if my topic has been been reviewed already?

Before starting out on a systematic review, check to see if someone has done it already. In PubMed you can use the systematic review subset to limit to a broad group of papers that is enriched for systematic reviews. You can invoke the subset by selecting if from the Article Types filters to the left of your PubMed results, or you can append AND systematic[sb] to your search. For example:

"neoadjuvant chemotherapy" AND systematic[sb]

The systematic review subset is very noisy, however. To quickly focus on systematic reviews (knowing that you may be missing some), simply search for the word systematic in the title:

"neoadjuvant chemotherapy" AND systematic[ti]

Any PRISMA-compliant systematic review will be captured by this method since including the words "systematic review" in the title is a requirement of the PRISMA checklist. Cochrane systematic reviews do not include 'systematic' in the title, however. It's worth checking the Cochrane Database of Systematic Reviews independently.

You can also search for protocols that will indicate that another group has set out on a similar project. Many investigators will register their protocols in PROSPERO , a registry of review protocols. Other published protocols as well as Cochrane Review protocols appear in the Cochrane Methodology Register, a part of the Cochrane Library .

• Next: Guides and Standards >>
• Last Updated: Feb 26, 2024 3:17 PM
• URL: https://guides.library.harvard.edu/meta-analysis

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings

Preview improvements coming to the PMC website in October 2024. Learn More or Try it out now .

• Advanced Search
• Journal List
• Indian J Dermatol
• v.59(2); Mar-Apr 2014

Understanding and Evaluating Systematic Reviews and Meta-analyses

Michael bigby.

From the Department of Dermatology, Harvard Medical School, Beth Israel Deaconess Medical Center, Boston, MA 02215, USA

A systematic review is a summary of existing evidence that answers a specific clinical question, contains a thorough, unbiased search of the relevant literature, explicit criteria for assessing studies and structured presentation of the results. A systematic review that incorporates quantitative pooling of similar studies to produce an overall summary of treatment effects is a meta-analysis. A systematic review should have clear, focused clinical objectives containing four elements expressed through the acronym PICO (Patient, group of patients, or problem, an Intervention, a Comparison intervention and specific Outcomes). Explicit and thorough search of the literature is a pre-requisite of any good systematic review. Reviews should have pre-defined explicit criteria for what studies would be included and the analysis should include only those studies that fit the inclusion criteria. The quality (risk of bias) of the primary studies should be critically appraised. Particularly the role of publication and language bias should be acknowledged and addressed by the review, whenever possible. Structured reporting of the results with quantitative pooling of the data must be attempted, whenever appropriate. The review should include interpretation of the data, including implications for clinical practice and further research. Overall, the current quality of reporting of systematic reviews remains highly variable.

Introduction

A systematic review is a summary of existing evidence that answers a specific clinical question, contains a thorough, unbiased search of the relevant literature, explicit criteria for assessing studies and structured presentation of the results. A systematic review can be distinguished from a narrative review because it will have explicitly stated objectives (the focused clinical question), materials (the relevant medical literature) and methods (the way in which studies are assessed and summarized).[ 1 , 2 ] A systematic review that incorporates quantitative pooling of similar studies to produce an overall summary of treatment effects is a meta-analysis.[ 1 , 2 ] Meta-analysis may allow recognition of important treatment effects by combining the results of small trials that individually might lack the power to consistently demonstrate differences among treatments.[ 1 ]

With over 200 speciality dermatology journals being published, the amount of data published just in the dermatologic literature exceeds our ability to read it.[ 3 ] Therefore, keeping up with the literature by reading journals is an impossible task. Systematic reviews provide a solution to handle information overload for practicing physicians.

Criteria for reporting systematic reviews have been developed by a consensus panel first published as Quality of Reporting of Meta-analyses (QUOROM) and later refined as Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA).[ 4 , 5 ] This detailed, 27-item checklist contains items that should be included and reported in high quality systematic reviews and meta-analyses. The methods for understanding and appraising systematic reviews and meta-analyses presented in this paper are a subset of the PRISMA criteria.

The items that are the essential features of a systematic review include having clear objectives, explicit criteria for study selection, an assessment of the quality of included studies, criteria for which studies can be combined, appropriate analysis and presentation of results and practical conclusions that are based on the evidence evaluated [ Table 1 ]. Meta-analysis is only appropriate if the included studies are conceptually similar. Meta-analyses should only be conducted after a systematic review.[ 1 , 6 ]

Criteria for evaluating a systematic review or the meta-analysis

A Systematic Review Should Have Clear, Focused Clinical Objectives

A focused clinical question for a systematic review should contain the same four elements used to formulate well-built clinical questions for individual studies, namely a Patient, group of patients, or problem, an Intervention, a Comparison intervention and specific Outcomes.[ 7 ] These features can be remembered by the acronym PICO. The interventions and comparison interventions should be adequately described so that what was done can be reproduced in future studies and in practice. For diseases with established effective treatments, comparisons of new treatments or regimens to established treatments provide the most useful information. The outcomes reported should be those that are most relevant to physicians and patients.[ 1 ]

Explicit and Thorough Search of the Literature

A key question to ask of a systematic review is: “Is it unlikely that important, relevant studies were missed?” A sound systematic review can be performed only if most or all of the available data are examined. An explicit and thorough search of the literature should be performed. It should include searching several electronic bibliographic databases including the Cochrane Controlled Trials Registry, which is part of the Cochrane Library, Medline, Embase and Literatura Latino Americana em Ciências da Saúde. Bibliographies of retrieved studies, review articles and textbooks should be examined for studies fitting inclusion criteria. There should be no language restrictions. Additional sources of data include scrutiny of citation lists in retrieved articles, hand-searching for conference reports, prospective trial registers (e.g., clinical trials.gov for the USA and clinical trialsregister.eu for the European union) and contacting key researchers, authors and drug companies.[ 1 , 8 ]

Reviews should have Pre-defined Explicit Criteria for what Studies would be Included and the Analysis should Include Only those Studies that Fit the Inclusion Criteria

The overwhelming majority of systematic reviews involve therapy. Randomized, controlled clinical trials should therefore be used for systematic reviews of therapy if they are available, because they are generally less susceptible to selection and information bias in comparison with other study designs.[ 1 , 9 ]

Systematic reviews of diagnostic studies and harmful effects of interventions are increasingly being performed and published. Ideally, diagnostic studies included in systematic reviews should be cohort studies of representative populations. The studies should include a criterion (gold) standard test used to establish a diagnosis that is applied uniformly and blinded to the results of the test(s) being studied.[ 1 , 9 ]

Randomized controlled trials can be included in systematic reviews of studies of adverse effects of interventions if the events are common. For rare adverse effects, case-control studies, post-marketing surveillance studies and case reports are more appropriate.[ 1 , 9 ]

The Quality (Risk of Bias) of the Primary Studies should be Critically Appraised

The risk of bias of included therapeutic trials is assessed using the criteria that are used to evaluate individual randomized controlled clinical trials. The quality criteria commonly used include concealed, random allocation; groups similar in terms of known prognostic factors; equal treatment of groups; blinding of patients, researchers and analyzers of the data to treatment allocation and accounting for all patients entered into the trial when analyzing the results (intention-to-treat design).[ 1 ] Absence of these items has been demonstrated to increase the risk of bias of systematic reviews and to exaggerate the treatment effects in individual studies.[ 10 ]

Structured Reporting of the Results with Quantitative Pooling of the Data, if Appropriate

Systematic reviews that contain studies that have results that are similar in magnitude and direction provide results that are most likely to be true and useful. It may be impossible to draw firm conclusions from systematic reviews in which studies have results of widely different magnitude and direction.[ 1 , 9 ]

Meta-analysis should only be performed to synthesize results from different trials if the trials have conceptual homogeneity.[ 1 , 6 , 9 ] The trials must involve similar patient populations, have used similar treatments and have measured results in a similar fashion at a similar point in time.

Once conceptual homogeneity is established and the decision to combine results is made, there are two main statistical methods by which results are combined: random-effects models (e.g., DerSimonian and Laird) and fixed-effects models (e.g., Peto or Mantel-Haenszel).[ 11 ] Random-effects models assume that the results of the different studies may come from different populations with varying responses to treatment. Fixed-effects models assume that each trial represents a random sample of a single population with a single response to treatment [ Figure 1 ]. In general, random-effects models are more conservative (i.e., random-effects models are less likely to show statistically significant results than fixed-effects models). When the combined studies have statistical homogeneity (i.e., when the studies are reasonably similar in direction, magnitude and variability), random-effects and fixed-effects models give similar results.

Fixed-effects models (a) assume that each trial represents a random sample (colored curves) of a single population with a single response to treatment. Random-effects models (b) assume that the different trials’ results (colored curves) may come from different populations with varying responses to treatment.

The point estimates and confidence intervals of the individual trials and the synthesis of all trials in meta-analysis are typically displayed graphically in a forest plot [ Figure 2 ].[ 12 ] Results are most commonly expressed as the odds ratio (OR) of the treatment effect (i.e., the odds of achieving a good outcome in the treated group divided by the odds of achieving a good result in the control group) but can be expressed as risk differences (i.e., difference in response rate) or relative risk (probability of achieving a good outcome in the treated group divided by the probability in the control group). An OR of 1 (null) indicates no difference between treatment and control and is usually represented by a vertical line passing through 1 on the x-axis. An OR of greater or less than 1 implies that the treatment is superior or inferior to the control respectively.

Annotated results of a meta-analysis of six studies, using random effects models reported as odd ratios using MIX version 1.7 (Bax L, Yu LM, Ikeda N, Tsuruta H, Moons KGM. Development and validation of MIX: comprehensive free software for meta-analysis of causal research data. BMC Med Res Methodol http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1626481/ ). The central graph is a typical Forest Plot

The point estimate of individual trials is indicated by a square whose size is proportional to the size of the trial (i.e., number of patients analyzed). The precision of the trial is represented by the 95% confidence interval that appears in Forest Plots as the brackets surrounding point estimate. If the 95% confidence interval (brackets) does not cross null (OR of 1), then the individual trial is statistically significant at the P = 0.05 level.[ 12 ] The summary value for all trials is shown graphically as a parallelogram whose size is proportional to the total number of patients analyzed from all trials. The lateral tips of the parallelogram represent the 95% confidence interval and if they do not cross null (OR of 1), then the summary value of the meta-analysis is statistically significant at the P = 0.05 level. ORs can be converted to risk differences and numbers needed to treat (NNTs) if the event rate in the control group is known [ Table 2 ].[ 13 , 14 ]

Deriving numbers needed to treat from a treatment's odds ratio and the observed or expected event rates of untreated groups or individuals

The difference in response rate and its reciprocal, the NNT, are the most easily understood measures of the magnitude of the treatment effect.[ 1 , 9 ] The NNT represents the number of patients one would need to treat in order to achieve one additional cure. Whereas the interpretation of NNT might be straightforward within one trial, interpretation of NNT requires some caution within a systematic review, as this statistic is highly sensitive to baseline event rates.[ 1 ]

For example, if a treatment A is 30% more effective than treatment B for clearing psoriasis and 50% of people on treatment B are cleared with therapy, then 65% will clear with treatment A. These results correspond to a rate difference of 15% (65-50) and an NNT of 7 (1/0.15). This difference sounds quite worthwhile clinically. However if the baseline clearance rate for treatment B in another trial or setting is only 30%, the rate difference will be only 9% and the NNT now becomes 11 and if the baseline clearance rate is 10%, then the NNT for treatment A will be 33, which is perhaps less worthwhile.[ 1 ]

Therefore, NNT summary measures within a systematic review should be interpreted with caution because “control” or baseline event rates usually differ considerably between studies.[ 1 , 15 ] Instead, a range of NNTs for a range of plausible control event rates that occur in different clinical settings should be given, along with their 95% confidence intervals.[ 1 , 16 ]

The data used in a meta-analysis can be tested for statistical heterogeneity. Methods to tests for statistical heterogeneity include the χ 2 and I.[ 2 , 11 , 17 ] Tests for statistical heterogeneity are typically of low power and hence detecting statistical homogeneity does not mean clinical homogeneity. When there is evidence of heterogeneity, reasons for heterogeneity between studies – such as different disease subgroups, intervention dosage, or study quality – should be sought.[ 11 , 17 ] Detecting the source of heterogeneity generally requires sub-group analysis, which is only possible when data from many or large trials are available.[ 1 , 9 ]

In some systematic reviews in which a large number of trials have been performed, it is possible to evaluate whether certain subgroups (e.g. children versus adults) are more likely to benefit than others. Subgroup analysis is rarely possible in dermatology, because few trials are available. Subgroup analyses should always be pre-specified in a systematic review protocol in order to avoid spurious post hoc claims.[ 1 , 9 ]

The Importance of Publication Bias

Publication bias is the tendency that studies that show positive effects are more likely to be published and are easier to find.[ 1 , 18 ] It results from allowing factors other than the quality of the study to influence its acceptability for publication. Factors such as the sample size, the direction and statistical significance of findings, or the investigators’ perception of whether the findings are “interesting,” are related to the likelihood of publication.[ 1 , 19 , 20 ] Negative studies with small sample size are less likely to be published.[ 1 , 19 , 20 ] Studies published are often dominated by the pharmaceutical company sponsored trials of new, expensive treatments often compared with the placebo.

For many diseases, the studies published are dominated by drug company-sponsored trials of new, expensive treatments. Such studies are almost always “positive.”[ 1 , 21 , 22 ] This bias in publication can result in data-driven systematic reviews that draw more attention to those medicines. Systematic reviews that have been sponsored directly or indirectly by industry are also prone to bias through over-inclusion of unpublished “positive” studies that are kept “on file” by that company and by not including or not finishing registered trials whose results are negative.[ 1 , 23 ] The creation of study registers (e.g. http://clinicaltrials.gov ) and advance publication of research designs have been proposed as ways to prevent publication bias.[ 1 , 24 , 25 ] Many dermatology journals now require all their published trials to have been registered beforehand, but this policy is not well policed.[ 1 ]

Language bias is the tendency for studies that are “positive” to be published in an English-language journal and be more quickly found than inconclusive or negative studies.[ 1 , 26 ] A thorough systematic review should therefore not restrict itself to journals published in English.[ 1 ]

Publication bias can be detected by using a simple graphic test (funnel plot), by calculating the fail-safe N, Begg's rank correlation method, Egger regression method and others.[ 1 , 9 , 11 , 27 , 28 ] These techniques are of limited value when less than 10 randomized controlled trials are included. Testing for publication bias is often not possible in systematic reviews of skin diseases, due to the limited number and sizes of trials.[ 1 , 9 ]

Question-driven systematic reviews answer the clinical questions of most concern to practitioners. In many cases, studies that are of most relevance to doctors and patients have not been done in the field of dermatology, due to inadequate sources of independent funding.[ 1 , 9 ]

The Quality of Reporting of Systematic Reviews

The quality of reporting of systematic reviews is highly variable.[ 1 ] One cross-sectional study of 300 systematic reviews published in Medline showed that over 90% were reported in specialty journals. Funding sources were not reported in 40% of reviews. Only two-thirds reported the range of years that the literature was searched for trials. Around a third of reviews failed to provide a quality assessment of the included studies and only half of the reviews included the term “systematic review” or “meta-analysis” in the title.[ 1 , 29 ]

The Review should Include Interpretation of the Data, Including Implications for Clinical Practice and Further Research

The conclusions in the discussion section of a systematic review should closely reflect the data that have been presented within that review. Clinical recommendations can be made when conclusive evidence is found, analyzed and presented. The authors should make it clear which of the treatment recommendations are based on the review data and which reflect their own judgments.[ 1 , 9 ]

Many reviews in dermatology, however, find little evidence to address the questions posed. The review may still be of value even if it lacks conclusive evidence, especially if the question addressed is an important one.[ 1 , 30 ] For example, the systematic review may provide the authors with the opportunity to call for primary research in an area and to make recommendations on study design and outcomes that might help future researchers.[ 1 , 31 ]

Source of Support: Nil

Conflict of Interest: Nil.

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings
• My Bibliography
• Collections
• Citation manager

Save citation to file

Email citation, add to collections.

• Create a new collection
• Add to an existing collection

Add to My Bibliography

Your saved search, create a file for external citation management software, your rss feed.

• Search in PubMed
• Search in NLM Catalog
• Add to Search

Easy guide to conducting a systematic review

Affiliations.

• 1 Discipline of Child and Adolescent Health, University of Sydney, Sydney, New South Wales, Australia.
• 2 Department of Nephrology, The Children's Hospital at Westmead, Sydney, New South Wales, Australia.
• 3 Education Department, The Children's Hospital at Westmead, Sydney, New South Wales, Australia.
• PMID: 32364273
• DOI: 10.1111/jpc.14853

A systematic review is a type of study that synthesises research that has been conducted on a particular topic. Systematic reviews are considered to provide the highest level of evidence on the hierarchy of evidence pyramid. Systematic reviews are conducted following rigorous research methodology. To minimise bias, systematic reviews utilise a predefined search strategy to identify and appraise all available published literature on a specific topic. The meticulous nature of the systematic review research methodology differentiates a systematic review from a narrative review (literature review or authoritative review). This paper provides a brief step by step summary of how to conduct a systematic review, which may be of interest for clinicians and researchers.

Keywords: research; research design; systematic review.

© 2020 Paediatrics and Child Health Division (The Royal Australasian College of Physicians).

PubMed Disclaimer

Similar articles

• Summarizing systematic reviews: methodological development, conduct and reporting of an umbrella review approach. Aromataris E, Fernandez R, Godfrey CM, Holly C, Khalil H, Tungpunkom P. Aromataris E, et al. Int J Evid Based Healthc. 2015 Sep;13(3):132-40. doi: 10.1097/XEB.0000000000000055. Int J Evid Based Healthc. 2015. PMID: 26360830
• Conducting systematic reviews of association (etiology): The Joanna Briggs Institute's approach. Moola S, Munn Z, Sears K, Sfetcu R, Currie M, Lisy K, Tufanaru C, Qureshi R, Mattis P, Mu P. Moola S, et al. Int J Evid Based Healthc. 2015 Sep;13(3):163-9. doi: 10.1097/XEB.0000000000000064. Int J Evid Based Healthc. 2015. PMID: 26262566
• Cervical spondylotic myelopathy: methodological approaches to evaluate the literature and establish best evidence. Skelly AC, Hashimoto RE, Norvell DC, Dettori JR, Fischer DJ, Wilson JR, Tetreault LA, Fehlings MG. Skelly AC, et al. Spine (Phila Pa 1976). 2013 Oct 15;38(22 Suppl 1):S9-18. doi: 10.1097/BRS.0b013e3182a7ebbf. Spine (Phila Pa 1976). 2013. PMID: 24026148
• What are the best methodologies for rapid reviews of the research evidence for evidence-informed decision making in health policy and practice: a rapid review. Haby MM, Chapman E, Clark R, Barreto J, Reveiz L, Lavis JN. Haby MM, et al. Health Res Policy Syst. 2016 Nov 25;14(1):83. doi: 10.1186/s12961-016-0155-7. Health Res Policy Syst. 2016. PMID: 27884208 Free PMC article. Review.
• What guidance is available for researchers conducting overviews of reviews of healthcare interventions? A scoping review and qualitative metasummary. Pollock M, Fernandes RM, Becker LA, Featherstone R, Hartling L. Pollock M, et al. Syst Rev. 2016 Nov 14;5(1):190. doi: 10.1186/s13643-016-0367-5. Syst Rev. 2016. PMID: 27842604 Free PMC article. Review.
• A Systematic Literature Review of Substance-Use Prevention Programs Amongst Refugee Youth. Aleer E, Alam K, Rashid A. Aleer E, et al. Community Ment Health J. 2024 Aug;60(6):1151-1170. doi: 10.1007/s10597-024-01267-6. Epub 2024 Apr 9. Community Ment Health J. 2024. PMID: 38592351 Free PMC article.
• The validity of electronic health data for measuring smoking status: a systematic review and meta-analysis. Haque MA, Gedara MLB, Nickel N, Turgeon M, Lix LM. Haque MA, et al. BMC Med Inform Decis Mak. 2024 Feb 2;24(1):33. doi: 10.1186/s12911-024-02416-3. BMC Med Inform Decis Mak. 2024. PMID: 38308231 Free PMC article.
• Methodological quality and reporting quality of COVID-19 living systematic review: a cross-sectional study. Luo J, Chen Z, Liu D, Li H, He S, Zeng L, Yang M, Liu Z, Xiao X, Zhang L. Luo J, et al. BMC Med Res Methodol. 2023 Jul 31;23(1):175. doi: 10.1186/s12874-023-01980-y. BMC Med Res Methodol. 2023. PMID: 37525117 Free PMC article.
• State of the Art of the Molecular Biology of the Interaction between Cocoa and Witches' Broom Disease: A Systematic Review. Santos AS, Mora-Ocampo IY, de Novais DPS, Aguiar ERGR, Pirovani CP. Santos AS, et al. Int J Mol Sci. 2023 Mar 16;24(6):5684. doi: 10.3390/ijms24065684. Int J Mol Sci. 2023. PMID: 36982760 Free PMC article. Review.
• Clarke M, Chalmers I. Reflections on the history of systematic reviews. BMJ Evid. Based Med. 2018; 23: 121-2.
• Oxman AD, Guyatt GH. The science of reviewing research. Ann. N. Y. Acad. Sci. 1993; 703: 125-33.
• Chalmers I, Hedges LV, Cooper H. A brief history of research synthesis. Eval. Health Prof. 2002; 25: 12-37.
• Lind J. A treatise of the scurvy. Three Parts Containing an Inquiry into the Nature, Causes and Cure, of that Disease Together with a Critical and Chronological View of what has been Published on the Subject. Edinburgh: Sands, Murray & Cochran; 1753. Available from: https://www.jameslindlibrary.org/lind-j-1753/ [accessed 16 February 2020].
• Oxman AD, Guyatt GH. Guidelines for reading literature reviews. CMAJ 1988; 138: 697-703.

Publication types

• Search in MeSH

Related information

Linkout - more resources, full text sources.

• Ovid Technologies, Inc.

• Citation Manager

NCBI Literature Resources

MeSH PMC Bookshelf Disclaimer

The PubMed wordmark and PubMed logo are registered trademarks of the U.S. Department of Health and Human Services (HHS). Unauthorized use of these marks is strictly prohibited.