title: 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.
authors.affiliation: Family Health International, PO Box 13950, Research Triangle Park, NC 27709, USA. dgrimes@fhi.org
authors.forename: David A
authors.initials: DA
authors.lastname: Grimes
authors.forename: Kenneth F
authors.initials: KF
authors.lastname: Schulz
mesh_terms: D000465:Algorithms; D016022:Case-Control Studies; D015331:Cohort Studies; D016001:Confidence Intervals; D004812:Epidemiologic Methods; D006801:Humans; D016032:Randomized Controlled Trials as Topic; D012106:Research / Q000145:classification*