## Definitions of Rates

For any given public health event (e.g., births, low birth weight births, mortality, hospitalizations) a county with a large population is likely to have more events per year than a county with a small population. Therefore, we calculate a rate instead of comparing the number of events.

A rate is a ratio of a public health event of interest to the population at risk of experiencing that public health event. Rates are calculated by dividing the number of events by the population at risk (or a related population) and then multiplying by a constant. The appropriate constant to use depends on the rarity of the event with the rarer events requiring a larger constant. For example, to calculate the infant death rate, divide the number of infant deaths in a year by the number of live births in that year and multiply by a constant of 1,000.

Example Formula: Infant Death Rate = (# of Infant Deaths / Live Births) * 1,000

To calculate the rate of low birth weight births in a year, divide the number of low birth weight infants by the number of live births that year and multiply by a constant of 100.

Example Formula: Low Birth Weight Rate = (# of Low Birth Weight Births / Live Births) * 1,000

Health indicators measuring mortality and hospitalization/ER rates are heavily influenced by age. For these indicators, rates are often age adjusted.

Indicators based on survey data use a weighted percent as the rate.