Counts 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 a rate will allow direct comparison between populations of unequal size that are otherwise similar (e.g., similar age composition, similar culturally).

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:

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 calculation for the crude death rate due to heart disease among males for 2004 looks like this:

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FAQs for Crude Rates:

Combining Years

Q: I am looking at death rates for a five-year period. What should I use for a population denominator?
A: If you are combining numerator values over the five years by summing them, then use the sum of the population counts over the same period. If you are combining numerator values by taking an average, then take an average of the population counts for the same time period and geographic area. Alternatively, you could also use an average over the five years in the numerator, and a "mid-point" population estimate, that is, a population estimate for the mid-point, or middle, year in the denominator.

An 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 greatest number of suicides occur among adolescents and young adults, whereas the highest rate occurs among elderly men.

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.

Always use estimates from the U.S. Census Bureau's Population Estimates Program (PEP), not the American Community Survey (ACS), when calculating health event rates. The exception is estimates for municipalities by age, sex, race, and/or Hispanic origin, which are not available from the PEP. Never use the April 1 estimates from the Decennial Census for rate calculations. More info.

If you're calculating a rate for:		Then use:
		Table/Dataset	File Name
NEW JERSEY
	Total population	State Population Totals	NST-ESTyyyy-POP
	By characteristics	Age, Sex, Race, and Hispanic Origin - 6 race groups	SC-ESTyyyy-ALLDATA6
COUNTIES
	Total population	County Population Totals	CO-ESTyyyy-POP
	By characteristics	Age, Sex, Race, and Hispanic Origin	CC-ESTyyyy-ALLDATA
MUNICIPALITIES
	Total population	Minor Civil Divisions	SUB-MCD-ESTyyyy-POP
	By characteristics	Go to data.census.gov. The ACS term for municipalities is "county subdivision."

yyyy = The most recent year available for the table or dataset
Always use the July 1 estimate from the year(s) that corresponds to your numerator.

The 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:

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