Please download our Microsoft Excel file to calculate Age-Adjusted Rates (XLS).
A crude death rate is a ratio of deaths during a year applied to the total mid-year population, e.g. 348.9 heart disease deaths per 100,000 population in 1997. However, populations used to compute rates often vary considerably according to age, race, sex and other demographic factors.
Therefore, if you want a measurement of mortality that can be used either to compare different populations (states, counties, cities, etc.) or to compare the mortality experience over time for one area with a changing population, it is advisable to adjust or standardize the effects of such factors as age and/or sex in these groups.
Death or incidence rates can be adjusted for any demographic factor such as race or any combination of factors, such as age, sex and race. The most commonly used adjustment - and the one we'll discuss here - is for age. Age-adjusted rates are commonly used in comparative mortality analyses since age is such a prime factor in mortality, especially with chronic diseases such as heart disease and diabetes. For example, counties with higher percentages of older residents will almost always have much higher crude death rates for all deaths as well as for major chronic conditions, compared to counties with younger populations. Conversely, counties with younger populations will tend to have higher crude rates for deaths due to unintentional injuries, especially motor vehicle crashes. Age-adjusted death rates eliminate the bias of age in the makeup of the populations being compared, thereby providing a much more reliable rate for comparison purposes.
We will use a method of adjusting called "direct standardization." It consists of applying specific crude rates to a standard population. The method serves the purpose of summarizing a set of specific rates independently of the characteristics of the population being studied.
There are three major components that are needed to perform adjusted mortality rate calculations:
- the number of deaths
- the population
- a "standard" population
These three components should be available by the demographic factors to which the rate is being adjusted. Since we are age-adjusting here, the figures must be available by age. The "standard population" listed above can be any population. The standard million age distribution of the total population of the U.S. according to the 1940 Census has been the most commonly used for adjustment of mortality rates. The National Center for Health Statistics (NCHS) and the Bureau of Health Statistics use it almost exclusively when calculating adjusted rates for reporting in their vital statistics publications. However, cancer incidence and mortality reports use the 1970 standard million U.S. population to conform to the National Cancer Institute's (NCI) calculations. Starting with 1999 events, NCHS and NCI will begin using a new standard population - the Year 2000 standard million population age distribution as projected by the Census Bureau in 1998. You must use the same standard population in the computation of any adjusted rates in order for the rates to be comparable.
The 1940 United States standard million population age distribution is as follows:
|All Ages ||1,000,000|
The year 2000 United States standard million population age distribution is as follows:
|All Ages ||1,000,000|
|Under 1 ||13,818|
Following is a step-by-step calculation of an age-adjusted county death rate (using the 1940 U.S. standard million population distribution):
Step 1 - List resident deaths and the population (for year/years being studied/compared) by the age categories for the county:
|<1 ||12 ||22,487 |
|85+||271 ||3,094 |
Step 2 - Calculate the age-specific rates by dividing Column A by Column B; e.g.. for the age group <1:
(Column A / Column B) = Column C
(12 / 22,487) = 0.000534
This is done for every age category in the population.
Step 3 - List the "standard" population as Column D. (Remember that it was decided to use the 1940 U.S. standard million population distribution.)
Step 4 - Multiply each crude rate (Column C) by the number in each of the corresponding age groups of the "standard" population (Column D).
|Column C||||Column D||||Column E|
|.000534||X||15,343 ||=||8.193 |
|.087589 ||X||2,770 ||=||242.622 |
The resultant figures (Column E) are the number of deaths to be "expected" in each age category if the specific death rates (Column C) had prevailed for one year within the "standard" population. These expected death figures must be calculated for all the age categories.
Below is an example of what we've done so far:
|<1 ||12 ||/||22,487 ||=||.000534||X||15,343 ||=||8.193 |
|85+ ||271 ||/||3,094||=||.087589 ||X||2,770 ||=||242.622|
Step 5 - Sum the expected deaths (Column E) and standard population (Column D) for all age categories.
Step 6 - To get the age-adjusted rate per 100,000 population, simply divide the total expected deaths (Column E) by the total standard population (Column D) and then multiply by 100,000:
(Total Column E / Total Column D) x 100,000 = (4916.748 / 1,000,000) x 100,000 = 491.7 =
age-adjusted death rate for County 1 per 100,000 1940 U.S. standard million population
These same steps using the same standard million population distribution must be followed when calculating the age-adjusted rates for any other counties to achieve comparability. Please also note that these basic steps are to be used regardless of what demographic factor/s you are adjusting for. For example, if you wanted to adjust the county rates for sex also, then columns A and B should have data for males or females. If you wanted to adjust only for race, then columns A and B should contain data by race.
We recommend that you download the Microsoft Excel spreadsheet using the links at the top of this page to calculate age-adjusted rates, especially if you will be calculating several or more rates.
Remember that an age-adjusted rate evens the differences in demographic factors such as age distribution but becomes an artificial figure in the process. Therefore, it is not an indicator of the absolute level of mortality in a population and is only useful for purposes of comparison. Some authorities contend that age-specific rates are more precise and useful than age-adjusted rates. See "Adjusted vs. Specific Rates."