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Distinguishing between "no evidence of effect" and "evidence of no effect" in randomised controlled trials and other comparisons
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Introduction |
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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.1 Despite this, many such "negative" trials,1 2 including many published in this journal, may wrongly be taken as evidence that the treatment is not clinically useful.
For example, in an RCT of women at risk of preterm delivery that
was not published as a full report,3 respiratory distress syndrome (RDS) occurred in three of 23 babies born to the treated group
and three of
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