# Rate

A measure of some event, disease, or condition in relation to a unit of population, along with some specification of time. (Also see Sources and Definitions, Age adjustment; Population.)

## Birth and related rates

**Birth rate:**

Calculated by dividing the number of live births in a population in a year by the midyear resident population. For census years, rates are based on unrounded census counts of the resident population as of April 1. For the noncensus years 1981–1989, rates are based on the midyear resident population, rounded to thousands. Rounded population estimates for 5-year age groups are calculated by summing unrounded population estimates before rounding to thousands. Starting in 1991, rates are based on unrounded national population estimates. (See Sources and Definitions, Population Census and Population Estimates.) Starting in 1997, the birth rate for the maternal age group 45–49 includes data for mothers ages 45 and older in the numerator and is based on the population of women ages 45–49 in the denominator. Birth rates are expressed as the number of live births per 1,000 population. The rate may be restricted to births to women of specific age, race, marital status, or geographic location (specific rate), or it may be for the entire population (crude rate).

**Fertility rate:** Total number of live births, regardless of the age of the mother, per 1,000 women of reproductive age (ages 15–44). Starting in 1997, the birth rate for the maternal age group 45–49 includes data for mothers ages 45 and older in the numerator and is based on the population of women ages 45–49 in the denominator.

## Death and related rates

**Death rate:** Calculated by dividing the number of deaths in a population in a year by the midyear resident population. For census years, rates are based on unrounded census counts of the resident population as of April 1. For the noncensus years 1981–1989, rates are based on the midyear resident population, rounded to thousands. Rounded population estimates for 10-year age groups are calculated by summing unrounded population estimates before rounding to thousands. Starting in 1991, rates are based on unrounded national population estimates. Rates for the Hispanic and White non-Hispanic populations through 1996 are based on unrounded population estimates for states in the reporting area for a Hispanic-origin item on the death certificate. (Also see Sources and Definitions, Hispanic origin: Mortality file.) Death rates are expressed as the number of deaths per 100,000 resident population. The rate may be restricted to deaths in specific age, race, sex, or geographic groups or from specific causes of death (specific rate), or it may be related to the entire population (crude rate). (Also see Sources and Definitions, Population Census and Population Estimates.)

**Fetal mortality rate:** Number of fetal deaths with stated or presumed gestation of 20 weeks or more, divided by the sum of live births plus fetal deaths, times 1,000. Late fetal mortality rate is the number of fetal deaths with stated or presumed gestation of 28 weeks or more, divided by the sum of live births plus late fetal deaths, times 1,000. (Also see Sources and Definitions, Fetal death; Gestation.)

**Infant mortality rate: **For infant mortality rates based on period files of linked birth/infant death data, rates are calculated by dividing the number of infant deaths during a calendar year by the number of live births reported in the same year. The rate is expressed as the number of infant deaths per 1,000 live births. Neonatal mortality rate is the number of deaths among infants younger than 28 days per 1,000 live births. Postneonatal mortality rate is the number of infant deaths that occur at ages 28 days to 364 days per 1,000 live births. For infant mortality rates based on the birth cohort linked birth/infant death data, rates are calculated as the number of deaths younger than age 1 year to members of the birth cohort, divided by the number of live births in a given year, times 1,000. (Also see Sources and Definitions, Birth cohort; Infant death.)

**Perinatal mortality rate: **Although several different perinatal mortality definitions exist, the perinatal definition used in *Health, United States* (and commonly used for international comparisons) is the sum of late fetal deaths at 28 weeks of gestation or more plus infant deaths younger than 7 days of birth, divided by the sum of live births plus late fetal deaths, times 1,000. The perinatal mortality ratio is the sum of late fetal deaths plus infant deaths younger than 7 days of birth, divided by the number of live births, times 1,000. (Also see Sources and Definitions, Fetal death; Gestation; Infant death.)

## Visit rate

**Visit rate:** A basic measure of service use for event-based data. Examples of events include physician office visits with drugs provided, or hospital discharges. In the visit rate calculation, the numerator is the number of estimated events, and the denominator is the corresponding U.S. population estimate for those who possibly could have had events during a given period. The interpretation is that for every person in the population, there were, on average, *x* events. It does not mean that *x* people in the population had events, because some people in the population had no events while others had multiple events. The only exception is when an event can occur just once for a person (for example, if an appendectomy is performed during a hospital stay). The visit rate is best used for comparing use across various subgroups of interest, such as age or race groups or geographic regions.