Ophthalmic statistics note 2: absence of evidence is not evidence of absence

Patients undergoing vitrectomy surgery for idiopathic full-thickness macular holes used to be routinely advised to follow a strict regime of posturing face down for a variable period (up to 2 weeks) after surgery.1–3 There was a scientific rationale for this—the tractional forces of gravity would force gases against the macula allowing it to heal more readily. Patients who postured were therefore believed to be less at risk of their macular hole reopening and of the need for repeat surgery to repair the hole. Medicine has clearly changed very significantly over time with a far greater emphasis on patient based outcomes and upon the need for an evidence base to justify practice.4 ,5 A senior colleague tells me that he has run a large randomised controlled clinical trial on patients who have had vitrectomies for macular holes. He states that the trial shows there is no difference in failure rates between patients who spent a week posturing face down after surgery and those who did not. He considers that this trial means that it is now unethical to ask patients to posture—particularly because several patients who did posture fed back to him how uncomfortable they found posturing. I ask him for a little more information about the trial and learn that it was a randomised controlled clinical trial with larger numbers of patients than typically found in ophthalmic surgical studies of 200 patients in each arm. Of those who spent a week posturing face down, one required repeat surgery. Of those who did not, two required repeat surgery. There is a published p value from a Fisher's exact test that was used to compare failure rates in the two groups of 0.999 and what seems to me to be an entirely cogent argument that this demonstrates no need for …