Health Event Rates
According 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.
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
Sex | Number of Deaths | Population Estimate | Crude Death Rate (Deaths per 100,000 Population) |
---|---|---|---|
Male | 9,598 | 4,235,853 | 226.6 |
Female | 10,966 | 4,463,026 | 245.7 |
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
The 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
Male | Female | |||||
---|---|---|---|---|---|---|
Age Group | Suicide Deaths | Population | Age- and Sex-Specific Rate per 100,000 Population | Suicide Deaths | Population | Age- and Sex-Specific Rate per 100,000 Population |
<15 | 5 | 897,553 | ** | 1 | 855,569 | ** |
15-44 | 250 | 1,821,036 | 13.7 | 56 | 1,792,745 | 3.1 |
45-64 | 155 | 1,038,488 | 14.9 | 37 | 1,112,479 | 3.3 |
65+ | 73 | 456,880 | 16.0 | 21 | 666,485 | 3.2 |
** Number is too small to calculate a reliable rate. |
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.
If the question is: | Then use: |
---|---|
MAGNITUDE: How big is the problem? | Number of events (count) |
PROBABILITY: What is the underlying risk in a population? | Crude rate and confidence interval |
DISPARITY: Is there a difference in risk after controlling for age? | Age-adjusted rate and confidence interval |