Radon enters the home through cracks and openings in the foundation. If the levels are only slightly elevated, simple measures such as caulking and fixing cracks may have the desired effect. Higher levels need more sophisticated remediation measures. This may include ventilating the soil underneath the home's foundation and venting the Radon gas to the atmosphere. This should be done by professionals since if the pressures are not correctly balanced, there is a possibility that fans could actually increase the amount of Radon in the home.
Epidemiologic studies try very hard to separate out the causes for the lung cancer.
This may be done by comparing rates of lung cancer in smokers versus nonsmokers and comparing Radon exposure levels to those with lung cancer versus those without lung cancer. Mathematically estimates can be made on a population basis of percentages of lung cancers attributable to cigarette smoking and percentages attributable to other causes such as Radon. I have heard of new homes being tested in the basement before all of the windows have been installed. Clearly this will not give an accurate estimation of the true Radon levels to the family after they move in. I heard of fans being installed in homes built in the 70's.