Observed vs. Expected Rates
An epidemiological method of comparing the actual (observed) number of disease cases in a community against the number that would be expected based on state or national averages. A statistically significant gap suggests an environmental cause.
Related terms
Epidemiology The branch of medicine that studies how often diseases occur in different groups of people and why. Epidemiologists are the disease detectives who investigate outbreaks and clusters. Statistically Significant A finding mathematically unlikely to be the result of random chance or coincidence. Used to confirm that a pattern—like a disease cluster—is real and not just noise in the data. State Cancer Registry A government-maintained database that tracks every newly diagnosed cancer case in a state, including type and location—essential for spotting cancer clusters. Cancer Cluster A greater-than-expected number of cancer cases occurring within a specific group of people in a defined geographic area over a certain period. The CDC's official designation requires intense, long-term statistical analysis.