In the March issue of BJO¹ a randomised controlled trial was reported in which topical versus sub-Tenon’s local anaesthesia for routine cataract surgery is compared. The trial is well designed and simple, using a validated outcome measure with the outcome observers masked to the intervention. It shows a clear preference for sub-Tenon’s block by patients undergoing routine cataract surgery.

Does this prove it? Is this sufficient evidence to change practice? In making such a decision, readers will want to critically appraise this evidence. Those who disagree with its findings will try hard to find flaws in the study while those that agree with it will tend to take it as face value and feel vindicated. Both sides will have to consider the probabilities that the results of the study could be erroneous or point to the truth.

What kinds of errors occur in trials? After bias and confounding have been dealt with (and by …