sentences:  In order to have firm evidence that some prospective intervention will have the desired beneficial effect on public health, the association observed between the particular risk factor and disease must imply that the risk factor either aggravates or actually "causes" the disease.Well-known successes include the identified causal links between smoking and lung cancer, and between blood pressure and stroke.However, there have also been notable failures when identified exposures were later shown by randomised controlled trials to be non-causal.