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Any casualties in the clash of randomised and observational evidence?

BMJ 2001; 322 doi: (Published 14 April 2001) Cite this as: BMJ 2001;322:879

No—recent comparisons have studied selected questions, but we do need more data

  1. John P A Ioannidis, associate professor and chairman (,
  2. Anna-Bettina Haidich, research fellow,
  3. Joseph Lau, professor
  1. Clinical Trials and Evidence-Based Medicine Unit, Department of Hygiene and Epidemiology, University of Ioannina School of Medicine, Ioannina 45110, Greece
  2. Division of Clinical Care Research, Department of Medicine, New England Medical Center, Tufts University School of Medicine, Boston, MA 02111, USA

    Randomised controlled trials and observational studies are often seen as mutually exclusive, if not opposing, methods of clinical research. Two recent reports, however, identified clinical questions (19 in one report,1 five in the other2) where both randomised trials and observational methods had been used to evaluate the same question, and performed a head to head comparison of them. In contrast to the belief that randomised controlled trials are more reliable estimators of how much a treatment works, both reports found that observational studies did not overestimate the size of the treatment effect compared with their randomised counterparts. The authors say that the merits of well designed observational studies may need to be re-evaluated: case-control and cohort studies may need to assume more respect in assessing medical therapies and largescale observational databases should be better exploited. 1 2 The first claim flies in the face of half a century of thinking, so are these authors right?

    The combined results from the two reports indeed show a striking concordance between the estimates obtained with the two research designs. A correlation analysis we performed on their combined databases found that the correlation coefficient between the odds ratio of randomised trials and the odds ratio of observational designs is 0.84 (P<0.001). This represents excellent concordance …

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