Power of the Cluster.

T he past 50 years have witnessed sea changes in cardio-vascular medicine and research. 1 We came to recognize untreated hypertension as a highly malignant disorder and to recognize that medical treatment of hypertension leads to marked reductions in risk of stroke, myocardial infarction, and premature death. We also came to recognize the power of the randomized trial for assessing candidate strategies for prevention and treatment. Indeed, mega-trials conducted by cardiovascular clinician scientists have played a critical role in defining nearly every major evidence-based intervention, from hypertension treatment to cholesterol reduction and use of aspirin. Over the past 10 to 20 years, we have also witnessed marked changes in the delivery of medical care. Instead of individual clinicians caring for individual patients, patients now increasingly find care within integrated systems with teams of providers. 2 Care ideally goes beyond even integrated healthcare clinics. In some countries and regions, there is increasing appreciation for expanding the sphere of medical care to communities. As medical care becomes more of a community or systems responsibility, as opposed to merely a single clinician responsibility, we are now coming to recognize the need for powerful methods to assess candidate community interventions, interventions that may come to implement and exploit the benefits of previously proven evidence based interventions. By definition, a healthcare system, or a community within which health care is given, cannot be the same as an individual patient. This may seem obvious yet has profound consequences for research strategies. It would make little sense to test a community-based intervention such as deployment of community-based health workers with a traditional random-ized trial that treats individual patients as the unit of study. Instead, we can take advantage of a long-standing but increasingly appreciated methodology: the cluster randomized trial. 5 In a cluster randomized trial, investigators randomize groups of people instead of individuals. They might random-ize hospital units, as was recently done in cluster trials of methicillin-resistant Staphylococcus aureus prevention 6 or of better implementation of internal mammary revasculariza-tion in patients undergoing coronary bypass grafting. 7 Or they might randomize whole communities, as in a recently reported trial on community health workers and enhanced physician education for the treatment of hypertension in Pakistan. 8 In this issue of Circulation, Tian and colleagues 9 report an intriguing randomized cluster trial of a " simplified cardio-vascular management program (SimCard) " for the management of patients at high risk for …