Users' Guides to the Medical Literature
Guyatt G, Rennie D, Meade MO, Cook DJ
Part B Therapy
Chapter 9.2. Surprising Results of Randomized Trials
Christina Lacchetti, John Ioannidis, Gordon Guyatt
Table 9.2-3 Refuted Evidence From Observational Studiesa
Topics Discussed:
aspirin, colorectal cancer, coronary artery bypass surgery, coronary heart disease, coronary revascularization, deep vein thrombosis, dementia, dexamethasone, diabetes mellitus, diuretics, duodenal ulcer, interpretation of results, ischemic stroke, leg ulcer, menopause, misleading results, myocardial infarction, observational studies, patient education, patient-important outcome, pulmonary embolism, randomized controlled trials, sodium fluoride, spinal fractures, statins
Excerpt:
"Table 9.2-3 demonstrates that the results
of observational studies are often an inadequate guide for therapeutic
decisions, even if they pertain to patient-important outcomes.
Some investigators have suggested that usually randomized and observational evidence agree
with similar evidence.57-59 An empirical evaluation,
however, examined 45 topics for which both RCTs and observational
studies were available on the same clinical question and used
the same outcome. Observational studies showed,
on average, larger benefits, and in 7 of these questions, the 2
designs gave results that were different beyond chance.60 Overall, observational
studies may be subject to more noise in their estimates compared
with randomized trials after accounting for differences
in sample size.61 Some observational studies may
use very large sample sizes (much larger than what randomized
trials can achieve), and therefore they produce spuriously
tight confidence intervals, whereas the true uncertainty
associated with their findings is much larger...."
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