Invited Article: Lost in a jungle of evidence

A 65-year-old woman presents with her third small subcortical stroke. This occurred despite treatment with antiplatelet medication for secondary stroke prevention. You have maximized risk-factor reduction and the patient has no high-grade carotid stenosis or source of cardiac embolism. You are considering warfarin to prevent another stroke. Looking for direction, you find one study that advocates this decision, while another seems to indicate that warfarin would be of no value in this situation.^1,2 Which study should you believe? If these studies have been reviewed and classified as part of an evidence-based guideline, the answer will be clear: The study that has been rated higher should be given more weight.

One of the essential features distinguishing guidelines from the American Academy of Neurology (AAN) is that the quality and risk of bias in a study is measured using a four-tiered classification-of-evidence scheme (table). It is important for clinicians to understand the classification scheme so that when they see a particular designation they understand its implications. In this scheme, studies graded Class I are judged to have high quality and low risk of bias; studies graded Class II are judged to have moderate quality and risk of bias; studies graded Class III are judged to have a high risk of bias; and studies graded Class IV are judged to have a very high risk of bias.

These classification schemes have been developed using empirically validated criteria for study strength. Although there are differences between the AAN classification scheme and those of other professional societies, they all essentially contain the same elements.³ For example, based upon the empirical observation that a randomized controlled trial is much more likely to give the correct answer than an observational study, all schemes require randomization for studies to …