Anne Abbott emphasises our finding that ipsi lateral stroke risk is dependent on the degree of asymptomatic carotid artery stenosis. Abbott states that this association was reported previously in the ACSRS study, on which she was an author. In fact, only a non-significant correlation towards increased stroke risk with degree of stenosis was found in the ACSRS study, but as we showed in our systematic review, other much earlier studies had already reported more convincing correlations. However, confusion was subsequently spread by the absence of any such associations in the randomised controlled trials of endarterectomy for patients with asymptomatic carotid stenosis. The purpose of our Article was to emphasise this discrepancy between cohort studies and trials and to explain that, although randomised controlled trials are the gold standard for quantifying the effects of an intervention, they are prone to selection biases that can undermine risk associations with in treatment groups.
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