## Health Event RatesThis page describes crude rates and age- and sex-specific rates for health events.## Crude RatesCounts of health events are useful, but have limitations for those who need to compare populations of unequal size, for instance, a subpopulation versus an overall state population. Knowing the population sizes can help to interpret counts, but computing aAccording to the dictionary, a rate is, "a quantity, amount, or degree of something [numerator], measured per unit of something else [denominator]." In public health, the numerator is the number of people among whom an event occurred during a certain period of time, and the denominator is the total number of people in the population at risk for the same period of time. A rate has four components: - A specified time period.
- The numerator, the number of people in whom an event occurred during a given period of time, and
- The denominator, the total number of people in the population at risk for the same period of time. This is also referred to as the "person-years at risk."
- A constant. The result of the fraction is usually multiplied by some factor of 10 (such as 100,000), so that the rate may be expressed as a whole number.
Many measures used in public health assessment specify a time period of one or more
In general, a rate is called a "crude rate" if it has not been adjusted for the age, race, ethnicity, sex, or other characteristic composition of a population. Table 1 shows an example of crude rate calculations for heart disease by sex. ## Table 1: Crude Death Rate due to Heart Disease by Sex, New Jersey, 2004
Using the values, above, for males as an example... - The specified time period is 2004.
- The numerator, or the number of events, was 9,598.
- The denominator, or the estimated population at risk, was the July 1, 2004 population estimate of 4,235,853.
- The constant was 100,000.
The calculation for the crude death rate due to heart disease among males for 2004 looks like this:
## FAQs for Crude Rates:## Combining Years## Age- and Sex-specific RatesAn age-specific rate is calculated by dividing the total number of health events for the specific age-group of interest by the total population in that age group. In Table 2, the age- and sex-specific rates for suicide are shown. The example demonstrates that the greatestThe calculation for an age-specific rate is the same as for a crude rate. ## Table 2: Suicide Mortality Rates by Age and Sex, New Jersey, 2004
Looking at rates within groups is also called "stratification." In Table 2, the population has been stratified
by age and sex. The data in Table 2 also show how useful stratification can be. Not only are the suicide
death rates much higher among men, the rate of suicide increases with age for men, but not for women.
The crude mortality rate for a population depends on the mortality
rate in each age group as well as on the proportion of people in each age group. For instance,
the age-specific rate for most causes of death will be higher for older age groups. As a result,
crude death rates tend to be higher in populations with a larger proportion of older persons,
and lower in populations with a larger proportion of younger persons. Age-specific rates are valuable for comparing rates across age groups, and crude rates provide a useful summary measure to compare similar populations of different sizes. But the word, "similar" is a key concept. It can be misleading to compare crude rates across populations that have relevant differences, such as different cultural traditions, or age, race/ethnicity, or sex composition. One difference that is commonly controlled for statistically is age composition of the population. The crude mortality rate for a population depends on the mortality rate in each age group as well as on the proportion of people in each age group. For instance, the age-specific rate for most causes of death will be higher for older age groups. As a result, crude death rates tend to be higher in populations with a larger proportion of older persons, and lower in populations with a larger proportion of younger persons. An age-adjusted rate is a summary measure that may be used to compare mortality or disease risk in two populations with different age compositions. ## Deciding Which Measure to UseThe measure that best informs the question you are trying to answer is the one to use. This is a guideline, not a hard and fast rule, but generally:
## Go to the page on age-adjusted rates for health events. |