Crude and Age-Adjusted Rate

Two types of rates are utilized in the MOPHIMS MICA system: crude and age-adjusted rates.

Crude Rates

A crude rate is defined as the total number of events, or count, divided by the mid-year total population of the selected geography and multiplied by a constant, which is a multiple of 10. Typical constants used for public health rates include 100, 1,000, 10,000, or 100,000.

Crude Rate = (Number of Events or Count ÷ Mid-year Population) x Constant

Crude rates are the only types of rates available in the Birth MICA, Pregnancy MICA, Fertility and Pregnancy Rate MICA, and the five WIC MICAs. Crude rates are reported in the Community Data Profiles for birth, pregnancy, and WIC indicators.  Rates for specific age groups, such as infants under the age of 1 or adults ages 20-24, are also crude rates unless otherwise specified.  (Age groups that cover a large range of ages, such as adults ages 18-64, may be age-adjusted.)

Age-adjusted Rates

Age adjusting rates allows fairer comparisons to be made between groups with different age distributions. For example, a county with a high percentage of elderly residents may have a higher rate of death or hospitalization than a county with a younger population, merely because the elderly are more likely to die or be hospitalized. (The same distortion can happen when comparing races, genders, or time periods.) Age adjustment can make the different geographies more comparable.

A "standard" population distribution is used to adjust rates that use population as the denominator for the rate calculation. (MICAs that use population as the denominator include:  Cancer Incidence, Chronic Disease Death, Chronic Disease Emergency Room, Chronic Disease Inpatient Hospitalization, Death, Emergency Room, Fertility and Pregnancy Rate, Injury, Inpatient Hospitalization, Preventable Hospitalization, and Procedures.) The age-adjusted rates are rates that would have existed if the population under study had the same age distribution as the "standard" population. Therefore, they are summary measures adjusted for differences in age distributions.

The National Center for Health Statistics recommends that the U.S. 2000 standard population be used when calculating age-adjusted rates. Users of the Missouri Information for Community Assessment (MICA) system have the option of selecting age-adjusted rates based on the U.S. 1940, 1970, or 2000 standard populations when generating tables where age-adjustment is utilized. However, if you compare rates from different sources, it is very important that the same standard population be used on both sides of a comparison. It is not legitimate to compare adjusted rates which use different standard populations.

Age-adjusted rates in the Community Data Profiles use the U.S. 2000 standard population.

Computation

An example of the computation of an age-adjusted death rate (AADR) follows:

The rate in the area of study (e.g., county, state) is computed for each age group noted in the table below by dividing the number of events (deaths) in that age group by the estimated population of the same age group in that area and then multiplying by a constant of 100,000. This results in an age-specific death rate (ASDR) per 100,000 population for each age group. That is, for each age group, ASDR = deaths in age group ÷ estimated population of that age group × 100,000.

Each ASDR is then multiplied by the proportion of the standard population (see table below) in that same age group. The age-specific results are summed to get the age-adjusted death rate for the area of study. The formula is:

AADR = Summation of (ASDR X standard proportion)

This is called the direct method of standardization. A given area's age-specific rate (overall or for a given cause) is applied to the U.S. standard population.

The following are the U.S. standard population distributions:

Age-adjusted rates published elsewhere (e.g., in the annual Missouri Vital Statistics) may be slightly different from those found in the MICAs or Community Data Profiles, due to updating of population estimates for years between decennial Censuses. The constant or "per population" number used for the age-adjusted rates may vary, depending on the type of event. For example, the age-adjusted rates for deaths are per 100,000 population. However, age-adjusted rates for hospitalizations and procedures are per 10,000 population and age-adjusted rates for emergency department visits are per 1,000 population.

References

1940 and 2000 U.S. standard populations acquired from:
Anderson RN, Rosenberg HM. Age Standardization of Death Rates: Implementation of the Year 2000 Standard. National Vital Statistics Reports; Vol. 47 No. 3. Hyattsville, Maryland: National Center for Health Statistics. 1998.

1970 U.S. standard population acquired from:
Devessa S, Grauman DJ, Blot WJ, Pennell GA, Hoover RN, Fraumeni JF Jr. Atlas of Cancer Mortality in the United States, 1950-94. Bethesda, Maryland: National Cancer Institute, 1999. NIH publication 99-4564.