What a systematic literature review is
A systematic literature review answers a single, focused research question by finding, evaluating and synthesising all the studies that meet pre-defined criteria, using an explicit and reproducible method. The defining words are systematic, transparent and reproducible: another researcher following your documented method should arrive at the same set of studies. That rigour is why well-conducted systematic reviews sit near the top of the evidence hierarchy, above individual studies and traditional reviews.
“A narrative review tells you what one author thinks the literature says; a systematic review shows you, transparently and reproducibly, what the literature actually says.”
— Why systematic reviews sit near the top of the evidence hierarchy
This is fundamentally different from a narrative (traditional) literature review, where an author selects and discusses sources at their discretion. A narrative review is broader and more flexible but also more subjective and prone to bias; a systematic review is narrower but far more rigorous and defensible. (For the general, narrative type of review — the kind most dissertations include as a chapter — see our literature review guide. This article is about the systematic method specifically.)
Why and when to do a systematic review
Systematic reviews originated in healthcare, where decisions must rest on the totality of evidence rather than one convenient study, and they remain the gold standard in medicine, nursing and public health. They have since spread to psychology, education, management, social policy and beyond. You would choose an SLR when your question is focused enough to answer comprehensively, when a body of primary studies already exists to review, and when transparency and reproducibility matter for your field.
For a student, a systematic review can be an excellent secondary-research dissertation: it requires no primary data collection or ethical approval, yet it demands real methodological rigour and produces a genuine contribution by synthesising what is known. It is not, however, an easy option — a full SLR is labour-intensive, and many student projects are better described as a ‘systematic-style’ or ‘structured’ review that borrows the method on a smaller scale. Be honest in your write-up about which you have done.
The systematic review process at a glance
The power of the method is its defined sequence of stages, each documented so others can follow it. The flow below shows the journey from question to synthesis.
Step 1: Define a focused question and protocol
Everything begins with a tightly focused question, because a systematic review answers one question exhaustively rather than surveying a broad area. In health and many social-science reviews the PICO framework structures the question into Population, Intervention, Comparison and Outcome — and variants exist for other fields (PEO, SPIDER for qualitative work). PICO does double duty: it sharpens the question and directly generates your search terms.
Question: In adults with type 2 diabetes, does a low-carbohydrate diet, compared with a standard diet, improve glycaemic control?
P (Population): adults with type 2 diabetes
I (Intervention): low-carbohydrate diet
C (Comparison): standard diet
O (Outcome): glycaemic control (e.g. HbA1c)
PICO turns a vague topic into a precise, searchable question and directly suggests your search terms and inclusion criteria.
Before searching, you write a protocol — a plan stating your question, search strategy, inclusion/exclusion criteria and methods of appraisal and synthesis. Writing the protocol in advance is what prevents you from cherry-picking studies to fit a desired conclusion; in formal reviews protocols are even registered (for example on PROSPERO). For a student review, a clear protocol agreed with your supervisor serves the same purpose.
Step 2: Build a reproducible search strategy
The search is the heart of a systematic review, and it must be documented well enough to repeat. Identify the key concepts in your question (from PICO), list synonyms and alternative terms for each, and combine them into Boolean search strings using AND (to combine concepts), OR (to combine synonyms) and sometimes NOT. For example: (‘type 2 diabetes’ OR T2DM) AND (‘low carbohydrate’ OR low-carb) AND (HbA1c OR ‘glycaemic control’).
Run the search across several relevant databases — for example PubMed/MEDLINE and CINAHL for health, PsycINFO for psychology, ERIC for education, Scopus and Web of Science across disciplines — because no single database covers everything. Record the exact search terms, databases, dates and number of hits for each, so the search can be reproduced and reported. This documentation is not bureaucratic box-ticking; it is the evidence that your review is systematic rather than a convenience sample of whatever you happened to find.
Step 3: Set inclusion and exclusion criteria
Decide in advance what makes a study eligible for your review, and apply the rules consistently. Typical criteria cover the population (who the studies are about), the study design (for example, only randomised controlled trials, or only qualitative studies), the date range, the language, the setting, and publication type (peer-reviewed only, excluding opinion pieces). Inclusion criteria say what you keep; exclusion criteria say what you discard and why.
Setting these before screening is what keeps the review objective: you are applying a rule, not choosing studies you like. State your criteria explicitly in the methodology so a reader understands exactly what your review does and does not cover — and so the boundaries of your conclusions are clear.
Step 4: Screen studies and report with PRISMA
Searches return far more results than are relevant, so you screen in stages. First remove duplicates. Then screen by title and abstract against your criteria, discarding the clearly irrelevant. Then retrieve and screen the remaining studies at full text, applying the criteria precisely. At each stage you record how many studies were included and excluded, and why.
This is reported visually in a PRISMA flow diagram (Preferred Reporting Items for Systematic Reviews and Meta-Analyses), the standard four-box flow — Identification, Screening, Eligibility, Included — that shows the numbers dropping at each step from thousands of hits to the handful of studies finally included. The PRISMA diagram and checklist are expected in health reviews and increasingly elsewhere; including them signals that your review meets recognised reporting standards. Where possible, a second reviewer screens a sample to check agreement and reduce bias.
Step 5: Appraise the quality of included studies
Not all studies are equally trustworthy, so you critically appraise the quality of each included study rather than treating them all as equal. Structured tools make this systematic: the CASP (Critical Appraisal Skills Programme) checklists are widely used and have versions for different study designs; other tools include the Cochrane risk-of-bias tool for trials and the JBI checklists. Appraisal asks whether the study’s design, sample, methods and analysis are sound, and how much weight its findings deserve.
Reporting your appraisal lets you weight the evidence in your synthesis — giving more credence to well-conducted studies and noting where conclusions rest on weaker ones. It also demonstrates the critical judgement examiners are looking for: a systematic review is not just a list of what studies found, but an assessment of how much we should believe them.
Step 6: Extract data and synthesise the findings
Finally, you extract the relevant data from each included study into a structured data-extraction table (author, year, design, sample, key findings, quality), and then synthesise across the studies. Synthesis takes two broad forms. Narrative synthesis groups and discusses findings thematically in words, identifying patterns, agreements and contradictions — suitable when studies are too varied to combine statistically. Meta-analysis statistically combines the results of comparable quantitative studies into a single pooled estimate — powerful, but only valid when the studies are similar enough to combine.
Synthesis is where the review earns its value: rather than summarising studies one by one, you draw them together to answer your question — what, overall, does the evidence show, how strong and consistent is it, and what gaps remain? That overarching, evidence-weighted answer is the contribution of a systematic review.
The most common systematic-review mistakes
- A question that is too broad. A systematic review answers one focused question exhaustively, not a wide topic.
- An undocumented search. If the search cannot be reproduced from your write-up, the review is not systematic.
- Setting criteria after screening. Decide inclusion/exclusion criteria in advance to avoid bias.
- No quality appraisal. Treating all studies as equally reliable ignores the evidence’s real weight.
- Summarising instead of synthesising. Draw the studies together to answer the question, do not list them one by one.
- Calling a small structured review a full SLR. Be honest about the scope of what you have done.
- Meta-analysing incomparable studies. Only pool results statistically when the studies are genuinely similar.
Choosing databases and including grey literature
A search is only as good as the sources it covers, so choosing the right databases matters. Use the major databases for your discipline — PubMed/MEDLINE and CINAHL in health, PsycINFO in psychology, ERIC in education, Business Source and ABI/INFORM in management — plus broad multidisciplinary indexes such as Scopus and Web of Science. Searching several databases is essential because each indexes a different, overlapping set of journals; relying on one, or on Google Scholar alone, risks missing relevant studies and undermines the ‘systematic’ claim.
Consider also grey literature — theses, reports, conference papers and working papers not published in journals — because limiting a review to published studies can introduce publication bias, the tendency for positive results to be published more readily than null ones. Searching trial registries, repositories and key organisations’ websites helps counter this. Whatever you search, record the databases, terms and dates precisely so the strategy can be reproduced exactly — that reproducibility is the whole point of a systematic search.
Adapting the method for a student timeline
A full systematic review by a research team can take a year or more, which is unrealistic for most undergraduate or master’s dissertations. The sensible response is to scale the method, not abandon it: keep the rigorous structure — focused question, documented search, explicit criteria, transparent screening, appraisal and synthesis — but limit the scope so it is achievable. You might restrict the date range, focus on one study design, or search several rather than a dozen databases.
Be honest in your write-up about what you have done. If you have followed systematic principles on a reduced scale, describe it as a ‘systematic-style’ or ‘structured’ review and state your limitations clearly. Examiners respect a well-executed, honestly described smaller review far more than an over-claimed ‘full systematic review’ the timeline could never have supported. Single-reviewer screening, rather than the two reviewers used in formal reviews, is normal for student work — just acknowledge it as a limitation that may introduce some selection bias.
Taking on a systematic literature review? Our researchers help you design the protocol, run reproducible searches, appraise studies and synthesise findings.
Related guides
- Dissertation writing services (hub)
- How to write a (narrative) literature review
- Primary vs secondary research
Frequently asked questions
What is the difference between a systematic review and a narrative literature review?
A narrative (traditional) review discusses sources the author selects at their discretion — broad but subjective. A systematic review follows a pre-defined, documented protocol to find, appraise and synthesise all studies meeting set criteria — narrower but transparent and reproducible. Most dissertation chapters are narrative reviews; a systematic review is a specific method.
What is PICO?
PICO is a framework for building a focused review question: Population, Intervention, Comparison and Outcome. It sharpens a vague topic into a precise, searchable question and directly suggests your search terms and inclusion criteria. Variants such as PEO and SPIDER suit other question types.
What is a PRISMA flow diagram?
PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) is a reporting standard. Its flow diagram shows, in four stages (Identification, Screening, Eligibility, Included), how the number of studies drops from the total search hits to the studies finally included, making the screening process transparent.
What is the difference between narrative synthesis and meta-analysis?
Narrative synthesis discusses and groups findings thematically in words, suitable when studies are too varied to combine. Meta-analysis statistically pools the results of comparable quantitative studies into a single estimate — more powerful, but only valid when the studies are similar enough to combine.
Can I do a systematic review for a student dissertation?
Yes — a systematic (or systematic-style) review makes an excellent secondary-research dissertation, needing no primary data or ethical approval but real rigour. Be realistic about scale: a full SLR is labour-intensive, and many student projects are better described honestly as a structured or systematic-style review.
Which databases should I search for a systematic review?
Use the major databases for your discipline — for example PubMed/MEDLINE and CINAHL for health, PsycINFO for psychology, ERIC for education — plus multidisciplinary indexes such as Scopus and Web of Science. Search several, because each indexes different journals, and consider grey literature to reduce publication bias.
What is publication bias?
Publication bias is the tendency for studies with positive or significant results to be published more readily than those with null or negative results. It can skew a review that searches only published journals, which is why systematic reviewers also search grey literature such as theses, reports and trial registries.
How long does a systematic review take?
A full systematic review by a team can take six months to well over a year. A student systematic-style review on a reduced scope can be completed within a dissertation timeline, provided the question is tightly focused and the search is sensibly limited. Plan the stages carefully, because screening in particular is time-consuming.
Can someone help me with my systematic review?
Yes — our researchers help design the protocol and search strategy, screen and appraise studies, build extraction tables and synthesise findings. See our dissertation writing services page or place an order.
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