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Comparison of evidence of treatment effects in randomized and nonrandomized studies
J P Ioannidis1, A B Haidich, M Pappa
1Division of Clinical Care Research, New England Medical Center, Box 63, 750 Washington St, Boston, MA 02111, USA. JLau1@lifespan.org
Context:
There is substantial debate about whether the results of nonrandomized studies are consistent with the results of randomized controlled trials on the same topic.
Objectives:
To compare results of randomized and nonrandomized studies that evaluated medical interventions and to examine characteristics that may explain discrepancies between randomized and nonrandomized studies.
Data Sources:
MEDLINE (1966-March 2000), the Cochrane Library (Issue 3, 2000), and major journals were searched.
Study Selection:
Forty-five diverse topics were identified for which both randomized trials (n = 240) and nonrandomized studies (n = 168) had been performed and had been considered in meta-analyses of binary outcomes.
Data Extraction:
Data on events per patient in each study arm and design and characteristics of each study considered in each meta-analysis were extracted and synthesized separately for randomized and nonrandomized studies.
Data Synthesis:
Very good correlation was observed between the summary odds ratios of randomized and nonrandomized studies (r = 0.75; P<.001); however, nonrandomized studies tended to show larger treatment effects (28 vs 11; P =.009). Between-study heterogeneity was frequent among randomized trials alone (23%) and very frequent among nonrandomized studies alone (41%). The summary results of the 2 types of designs differed beyond chance in 7 cases (16%). Discrepancies beyond chance were less common when only prospective studies were considered (8%). Occasional differences in sample size and timing of publication were also noted between discrepant randomized and nonrandomized studies. In 28 cases (62%), the natural logarithm of the odds ratio differed by at least 50%, and in 15 cases (33%), the odds ratio varied at least 2-fold between nonrandomized studies and randomized trials.
Conclusions:
Despite good correlation between randomized trials and nonrandomized studies-in particular, prospective studies-discrepancies beyond chance do occur and differences in estimated magnitude of treatment effect are very common.
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