The attempt to determine the relative importance of life events that contribute to functional impairments. Determining the relationship between occupational or other exposures and the development of chronic illness is often a complex task. For example, chronic obstructive lung diseases may result from non-occupational factors such as tobacco abuse, or from genetic illnesses such as alpha,-antitrypsin disease or from job-related exposures to chemicals, dusts, or asbestos. Similarly, hearing loss may occur as a natural consequence of aging or as a result of exposures to high levels of noise at work (e.g., with heavy machinery) or during recreation (motorcycle riding or snowmobiling).