Grey literature is research produced outside the channels of commercial or academic publishing: theses and dissertations, government and agency reports, conference abstracts, clinical trial registry entries, working papers, and organisational documents. It is "grey" because it is neither formally published in a peer-reviewed journal (the "white" literature) nor entirely hidden, but it sits in a harder-to-search middle ground. In a systematic review, searching it matters because the studies that never reach a journal are often the ones with null or unfavourable results.
Ignoring grey literature does not make a review cleaner; it makes it biased toward whatever happened to get published. That is why comprehensive search guidance treats grey literature as a planned source rather than an afterthought.
Why grey literature fights publication bias
Studies with statistically significant, positive findings are more likely to be submitted and accepted by journals than studies with null results. If a review searches only journal databases, it inherits that skew, and the pooled effect can look stronger than the true picture. Searching grey literature, where many null and unfavourable results live, is one of the main defences against this publication bias. This is the same concern a meta-analysis probes with funnel plots, and the two protections work together: a thorough search reduces the bias, and the analysis checks for what remains.
Because the goal is comprehensiveness, the decision to search grey literature should be made and justified in the protocol, alongside the rest of your database search strategy, not improvised once the journal search disappoints.
What counts, and where to look
The category is broad, so it helps to map common types to where you actually find them.
| Type of grey literature | Where to search |
|---|---|
| Theses and dissertations | ProQuest Dissertations, EThOS, national repositories |
| Clinical trial records | ClinicalTrials.gov, WHO trial registries |
| Government and agency reports | Department and agency websites, policy databases |
| Conference abstracts | Conference proceedings, Embase abstract records |
| Preprints and working papers | Preprint servers, institutional repositories |
Beyond these sources, two techniques catch what database searches miss: citation searching (following the reference lists of included studies backward, and the studies that cite them forward) and targeted website searching. Both are reportable steps, and the modern flow diagram has a dedicated arm for records found through these non-database routes.
Is grey literature credible enough to include?
Credibility is the usual objection, and the answer is that grey literature is assessed for quality exactly like any other study, not trusted or dismissed on its source alone. A government report from a national statistics agency and an unrefereed blog post are both technically grey, but only one survives a risk of bias assessment. The same eligibility criteria and quality appraisal apply, so weak grey sources are excluded on their merits rather than pre-emptively banned.
The honest position is that excluding all grey literature to avoid low-quality sources throws out the valuable reports along with the weak ones, and reintroduces the very publication bias the review is meant to limit.
Documenting it in the flow diagram
Every grey-literature record must be traceable through the same process as journal records: counted at identification, de-duplicated, screened, and either included or excluded with a reason. In the PRISMA 2020 layout these records flow through the other sources column rather than the database column, which keeps the two routes visibly separate. The counts you record there feed straight into the diagram, which you can assemble with our free PRISMA flow diagram generator once screening is done. For the full review workflow that surrounds this step, see our step-by-step systematic review guide.
Frequently Asked Questions
Can you use grey literature in a systematic review?
Yes, and comprehensive reviews are expected to. Searching grey literature such as theses, trial registries, and government reports reduces publication bias by capturing studies that never reached a peer-reviewed journal. Grey-literature records are screened against the same eligibility criteria and quality assessment as journal articles, and they are documented in the flow diagram through the other-sources route.
What is an example of grey literature?
Common examples include doctoral theses and dissertations, clinical trial registry entries, government and agency reports, conference abstracts, preprints, working papers, and organisational policy documents. What unites them is that they are not formally published in commercial peer-reviewed journals, yet they often contain relevant data, including null results that journals are less likely to publish.
Is grey literature credible?
Grey literature varies widely in credibility, so it is appraised case by case rather than trusted or rejected on its source alone. A national agency report and an unrefereed online post are both grey, but only the rigorous source survives a formal risk of bias assessment. Applying the same quality criteria used for journal articles keeps weak grey sources out while retaining valuable ones.
What is grey literature versus white literature?
White literature is research formally published in peer-reviewed, commercially indexed journals and books. Grey literature is everything produced outside those channels, such as theses, reports, and conference material, that is still publicly available but not peer-reviewed in the conventional sense. Searching both gives a systematic review a more complete and less biased evidence base.