Distinguishing between “no evidence of effect” and “evidence of no effect” in randomised controlled trials and other comparisons

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  • Format Texts
  • Language/s English
  • Target Audience Further education
  • EBM Stage 3 - Appraising evidence, 4 - Decision making
  • Duration <5 mins
  • Difficulty Introductory, Intermediate

Key Concepts addressed


If a court fails to convict a defendant because of incomplete
evidence, does that establish his innocence beyond doubt? Not necessarily. Indeed, in Scotland, if sufficient uncertainty remains, the court can give a verdict of “not proven” instead of “not guilty”. If a randomised controlled trial (RCT) fails to show a significant difference between the treatment and the control group, does that prove that the treatment has no useful clinical effect? Again, not necessarily. The treatment may work, but the trial may have been unable to prove it. Despite this, many such “negative” trials, including many published in this journal, may wrongly be taken as evidence that the treatment is  not clinically useful.

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