Nature and role of observational studies in public health policy concerning the effects of dietary salt intake on blood pressure

Does dietary sodium in excess of a specified daily consumption substantively elevate blood pressure in normotensive people? Does lowering the daily consumption of sodium reduce blood pressure in normotensive people? Sound observational and interventional studies can address these questions, but there are substantial differences in the ability of various research designs to provide clear, bias-free answers. I summarize established scientific principles for addressing issues of causation and the effects of interventions and compare observational and interventional designs. Observational studies are important in exploring the possible determinants of health problems but are subject to bias and cannot directly assess the effects of interventions. They are superseded by sound interventional studies, particularly randomized controlled trials, in answering the key questions concerning causes and benefits of intervention.