It's important to ask a question that can be answered through use of evidence rather than subjective judgment. Evidence-based medicine (EBM) - like evidence-based practice in any discipline - draws on empirical evidence. This may include laboratory studies, clinical trials, and real-world prospective or retrospective studies. You may need to rephrase the question according to whether the evidence base is in diagnosis, therapy & interventions, prognosis, or assessments of harm.
EBM gives more weight and authority to randomised controlled clinical trials and cohort studies than case studies. It gives more weight again to meta-analyses synthesising the results of many RCTs.
Figure 1: The hierarchy of evidence
Greenhalgh (1997) gives an alternative ranking of the types of primary study :
(1) Systematic reviews and meta-analyses
(2) Randomised controlled trials with definitive results (confidence intervals that do not overlap the threshold
clinically significant effect)
(3) Randomised controlled trials with non-definitive results (a point estimate that suggests a clinically
significant effect but with confidence intervals overlapping the threshold for this effect)
(4) Cohort studies
(5) Case-control studies
(6) Cross sectional surveys
(7) Case reports.
It's important to ask a question in enough detail to get relevant and appropriate results.
You may have a specific research question to address. A concept map to explore key terms likely to feature in your target literature - with synonyms and related terms - is often helpful.
Or you may be dealing with one or more patients in clinical casework. A clinical question has a number of essential facets, outlined in the PICO(TS) mnemonic.
P - Patient (diagnosis, age etc); Population
I - Intervention; Incident; Exposure
C- Control group; Comparison
O - Outcome; Effect
T - Timeframe or duration (optional)
S - Type of Study (optional)
e.g. In paediatric patients with psoriasis, is a topical corticosteroid or a systemic treatment such as methotrexate safer and more effective in the long term?
An alternative search strategy tool for qualitative/mixed methods research has been suggested: the SPIDER mnemonic.
PI- Phenomenon of Interest
R- Research type
How do you intend to analyse and report your findings?
The type of knowledge synthesis you pursue may vary with your field of research and research questions, and whether you are looking for quantitative, qualitative or mixed methods studies.
Meta-analysis collates statistical evidence from randomised controlled trials. Your methodology may follow a protocol such as the Cochrane Collaboration or PRISMA guidelines. PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) also stipulates details of flowcharts and recordkeeping for your workflows in searching and critically appraising your results.
If you are interested in qualitative research - perhaps using a post-positivist or interpretivist lens - you may be aiming for a narrative or realist synthesis. The RAMESES protocols ( Realist And Meta-narrative Evidence Syntheses: Evolving Standards) will be more appropriate.
Best-fit framework synthesis is suited to policy questions. It consists of one or more scoping reviews to assess the policy landscape before finalising the search strategy with the most effective concepts and criteria.
PRISMA is a checklist - an evidence-based minimum set of items - for reporting in systematic reviews and meta-analyses. Its purpose is to improve the reporting of systematic reviews and meta-analyses. It may also assist in the critical appraisal of systematic reviews.
The PRISMA checklist aligns with the usual sections of a research report or published paper. It stipulates information for inclusion in the title, abstract, introduction, methods, results, discussion, and funding sections.
The PRISMA flow-chart represents the workflow in critical appraisal as a trail tracking the items retrieved in the initial systematic literature search through their inclusion or exclusion from the study.
Figure 2. PRISMA flow-chart