An overview of clinical research: the lay of the land
Abstract
Many clinicians report that they cannot read the medical literature critically. To address this difficulty, we provide a primer of clinical research for clinicians and researchers alike. Clinical research falls into two general categories: experimental and observational, based on whether the investigator assigns the exposures or not. Experimental trials can also be subdivided into two: randomised and non-randomised. Observational studies can be either analytical or descriptive. Analytical studies feature a comparison (control) group, whereas descriptive studies do not. Within analytical studies, cohort studies track people forward in time from exposure to outcome. By contrast, case-control studies work in reverse, tracing back from outcome to exposure. Cross-sectional studies are like a snapshot, which measures both exposure and outcome at one time point. Descriptive studies, such as case-series reports, do not have a comparison group. Thus, in this type of study, investigators cannot examine associations, a fact often forgotten or ignored. Measures of association, such as relative risk or odds ratio, are the preferred way of expressing results of dichotomous outcomes-eg, sick versus healthy. Confidence intervals around these measures indicate the precision of these results. Measures of association with confidence intervals reveal the strength, direction, and a plausible range of an effect as well as the likelihood of chance occurrence. By contrast, p values address only chance. Testing null hypotheses at a p value of 0.05 has no basis in medicine and should be discouraged.
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Systematic reviews of observational studies have limited value for ascertaining causality.
“A number of limitations however should be highlighted starting with the limited value of systematic reviews of observational studies for ascertaining causality.81”
From Periodontitis is associated with hypertension: a systematic review and meta-analysis (2020) · cites this paper as [81]
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- Authors
- Grimes, D. A., & Schulz, K. F.
- Affiliations
- Family Health International 360 (US)
- Corresponding author
- Grimes, David A
- Venue
- The Lancet
- Publisher
- Elsevier BV
- Published
- 2002-01
- Volume
- 359
- Issue
- 9300
- Pages
- 57-61
- ISSN
- 0099-5355 (ISSN-L)
- 0140-6736
- OpenAlex
- W2163223903
- Cited by
- 734
- References
- 40
- Citations by year
- 2026: 36 · 2025: 35 · 2024: 50 · 2023: 44 · 2022: 48 · 2021: 44 · 2020: 42 · 2019: 41 · 2018: 29 · 2017: 32 · 2016: 26 · 2015: 26 · 2014: 29 · 2013: 27 · 2012: 33
- FWCI
- 4.56
- Type
- journal article
- Language
- en
- Open access
- closed
- Locations
- The Lancet (publishedVersion)
- PubMed (publishedVersion)
- DigitalGeorgetown (Georgetown University Library) (submittedVersion)
- The Lancet
- Topics
- Physical Sciences › Mathematics › Statistics and Probability › Statistical Methods in Epidemiology
- Social Sciences › Decision Sciences › Statistics, Probability and Uncertainty › Meta-analysis and systematic reviews
- Health Sciences › Health Professions › Health Information Management › Methodologies in Health Research and Practice
- Keywords
- clinical research, research design, randomized controlled trials, observational studies, cohort studies, case-control study, cross-sectional study, case series, association measures, relative risk, odds ratio, confidence intervals, p-values, null hypothesis testing, critical appraisal of medical literature
- MeSH terms
- Algorithms, Epidemiologic Methods, Humans, Research, Cohort Studies, Confidence Intervals, Case-Control Studies, Randomized Controlled Trials as Topic
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