Strata Academy

Grey Literature & Thesis Searching for Systematic Reviews

Conference abstracts, theses, regulatory documents, preprints, and when grey literature search changes review conclusions

Quick answer

Grey literature includes material not published in mainstream indexed journals: theses, conference abstracts, regulatory reports, government documents, and preprints. Cochrane methods recommend pre-specifying grey literature sources in the protocol, searching trial registries and key repositories, and reporting what was searched in PRISMA. Grey literature search reduces but does not eliminate publication bias.

1. What counts as grey literature

Grey literature is research output not controlled by commercial publishers: dissertations, conference proceedings, technical reports, regulatory submissions, policy documents, and organisational white papers. It sits outside standard bibliographic databases like MEDLINE.

The term is imprecise but methodologically important. Positive results publish in journals; null or inconvenient findings often remain in conference posters, student theses, or registry records. Ignoring grey sources can inflate apparent treatment effects in meta-analyses.

Grey literature is not synonymous with low quality. A well-conducted MSc thesis may contain more detail than a 300-word conference abstract. Appraise each record on its merits using the same eligibility and risk-of-bias criteria as published trials.

Preprints (manuscripts posted before peer review) occupy a grey zone. They may later publish as journal articles — deduplicate carefully. Preprint searching is increasingly relevant in fast-moving fields but requires explicit protocol justification.

2. Why grey literature matters for bias

Publication bias occurs when studies with statistically significant or favourable results are more likely to appear in indexed journals. Funnel plot asymmetry in meta-analysis is one signal; searching grey sources is one mitigation.

Cochrane Handbook Chapter 4 notes that including grey literature may increase the number of included studies and shift pooled estimates toward null. The direction of change is unpredictable — which is precisely why comprehensive searching matters.

For student reviews, supervisors often ask whether you addressed unpublished work. A documented grey literature plan, even if yield is modest, demonstrates awareness of synthesis limitations.

3. Where to search

Trial registries are the highest-yield grey source for intervention reviews. ClinicalTrials.gov, ISRCTN, EU Clinical Trials Register, and WHO ICTRP should be searched with terms parallel to your database strategy. Compare registry-recorded outcomes to published reports for selective reporting.

Conference proceedings: Embase indexes many conference abstracts; additionally search conference websites or proceedings repositories for specialty meetings relevant to your topic (e.g. ESC, ASCO, EULAR).

Thesis repositories: ProQuest Dissertations & Theses Global, EThOS (British Library), and institutional repositories. OpenGrey aggregates European grey literature. Google Scholar is supplementary — not reproducible as a sole source — but useful for locating reports and theses.

Regulatory and industry sources: FDA Drugs@FDA, EMA EPAR documents, and Clinical Study Reports where accessible (often limited for students). Justify scope in the protocol.

4. Building a grey literature search strategy

Mirror database search concepts where platforms support keyword search. Trial registries accept simpler strings than Ovid MEDLINE — often population and intervention terms without MeSH.

For thesis searching, use ProQuest advanced search with publication type filter 'Dissertation/Thesis' and concept blocks aligned to PICO. EThOS provides free access to many UK theses.

Set pragmatic limits. Exhaustive handsearching of every conference in a field is rarely feasible for student timelines. Pre-specify which conferences, date ranges, and repositories you will search; acknowledge what was not searched as a limitation.

Document search dates and export counts. Grey literature yields are often small but methodologically significant — one unpublished RCT can change a meta-analysis conclusion.

5. Appraising and including grey records

Conference abstracts frequently lack methods detail. Apply the same exclusion criteria: if design, population, or intervention cannot be verified, exclude with reason 'insufficient information' rather than assuming quality.

Link duplicate reports: a conference abstract may correspond to a later journal publication or registry record. Use trial registry IDs and author names to merge records before extraction.

Risk-of-bias assessment applies equally. ROB 2 for randomised trials in theses; ROBINS-I for observational grey reports. Missing methods domains should be rated unclear or high risk, not assumed low risk because the source is unpublished.

In meta-analysis, consider sensitivity analyses excluding grey literature records to assess robustness. Report whether pooled estimates change materially.

6. Reporting grey literature in PRISMA

PRISMA 2020 Item 7 lists all information sources searched. PRISMA-S provides structured fields for supplementary searches including grey literature repositories and trial registries.

Report the number of records identified from each grey source separately before deduplication. Note how many proceeded to full-text assessment and how many were included.

If no grey literature was searched, state this explicitly as a limitation — do not imply comprehensive searching. AMSTAR 2 Item 4 asks whether the search included grey literature databases and trial registries.

In the discussion, interpret how missing unpublished data might affect conclusions. GRADE certainty may be downgraded for publication bias when funnel plot asymmetry or registry-discrepancy suggests selective reporting.

Interactive version (quizzes, walkthroughs) loads when JavaScript is enabled.