Systematic reviews make some of the best scientific evidence. Reviewing the entire literature on a given issue filters out the glare of individual studies, giving researchers a better sense of the big picture — and giving people in clinical fields, such as nurses, better guidance about how to treat patients. (Britain's famed Cochrane reviews, for example, look at everything from treatment for back pain to the therapies for hepatitis C infection.) The problem comes when these systematic reviews include faulty studies.