Radon is a radioactive gas that seeps into homes from the soil. it is thought to be a leading cause of lung cancer. a research team investigates this theory. they gather large amounts of data on basement radon concentrations and lung cancer rates and conclude that the more radon there is, the more likely lung cancer is to occur. after the study is published, other researchers criticize it by asserting that the studied neighborhoods with higher radon concentrations also have a higher percentage of old people and a higher percentage of cigarette smokers than the low-radon neighborhoods. both advanced age and cigarette smoking increase the risk of lung cancer. this criticism, if correct, shows that the radon study suffered from