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 22 babies born to the untreated group. The difference is not significant (2p > 0.9). Had this been the first and only study of this treatment, many people might have decided that it was not effective and thus lost interest. In fact, overviews of this,3 and at least 14 other trials, eventually showed that the treatment—antenatal steroids—is highly effective because it reduced RDS and neonatal mortality in over 3500 preterm infants by about half.4 5 Note that the results of the single trial were quite consistent with this finding.3 The correct conclusion from that single trial …