A study that compares large groups of people instead of individuals for differences in things such as cancer rates. The groups can differ by location (for example, city, county, or country). They can also differ by time (a few days, years, or decades). Groups can be immigrants (compared with people who are native to the country) or people with different types of jobs. The Surveillance, Epidemiology, and End Results (SEER) Program conducts ecologic studies to collect information on cancer rates over time in certain parts of the United States. Also called ecological study.