Studying the incidence of a disease in a specific group of people can lead to the discovery of causation- what makes it occur. For example,the high incidence of lung cancer among cigarette smokers suggests that smoking is a causative factor in the development of lung cancer. However, care must be taken not to assume every association between a disease and a measured variable is causative. To say there is an association between two things simply means that they occur togerther in a certain situation. For example, several studies have indicated high rates of lung cancer in cook. Although this could be a consequence of their work