Choudry and colleagues' paper in this issue (p 141) is a brave attempt to quantify the under-recognised notion of personal knowledge in clinical practice.¹² They carefully analysed the prescribing decisions of hospital doctors caring for patients with atrial fibrillation, before and after their exposure to a patient with an adverse event—either serious haemorrhage when taking warfarin, or a thromboembolic event while not taking warfarin. The researchers wanted to know whether doctors' knowledge of a previous adverse event affected their subsequent prescribing.

For the group of doctors who were exposed to patients with adverse bleeding events, and who cared for atrial fibrillation patients subsequently, the odds that they would prescribe warfarin were 21% lower for subsequent patients. There was no statistically significant change in warfarin prescribing after a doctor cared for a patient …