There is considerable uncertainty regarding the risk estimates for low levels of radiation exposure as commonly experienced in diagnostic radiology procedures. This is because the risk is quite low compared to the natural risk of cancer. At low doses, the radiation-related excess risk, which is thought to be proportional to dose, tends to be dwarfed by statistical and other variation in the background risk level. To obtain adequate evidence for a statistically valid estimate of cancer risk from exposure to low doses of radiation would require studying millions of people for many years. There are some who question whether there is adequate evidence for a risk of cancer induction at low doses. Some scientists believe that low doses of radiation do not increase the risk of developing cancer at all, but this is a minority view.