NJSHAD - Age-Adjusted Rates

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An age-adjusted rate is a measure that controls for the effects of age differences on health event rates. When comparing across geographic areas, some method of age-adjusting is typically used to control for the influence that different population age distributions might have on health event rates.

Crude rates provide a useful summary measure to compare similar populations of different sizes, but crude rates are sensitive to differences in age compositions.

For example, a county with an older population will have a higher crude death rate due to cancer, even though its risk exposure levels and age-specific cancer rates may the same as those in other counties. One might incorrectly attribute the high cancer rate to some characteristic of the county other than age. Age-adjustment may also be used to control for age effects when comparing across several years of data, as the age distribution of the population changes over time. Calculating age-adjusted rates may be accomplished using direct, or indirect age standardization.

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Direct Age-adjustment

Direct age-adjustment (or age standardization) is the same as calculating a weighted average. It weights the age-specific rates observed in a population of interest by the proportion of each age group in a standard population (Lilienfeld & Stolley).

In 1999, the Centers for Disease Control and Prevention (CDC) replaced the 1940 U.S. standard population weights that had been used for the several previous decades with revised standard population weights for direct age-adjustment (Klein & Schoenborn). Table 1, below, contains the CDC standard population weights, which represent the proportion of the U.S. 2000 population in each age group, and sum to 1.0.

Compare only age-adjusted rates that have been adjusted to the same standard population. For instance, don't compare rates age-adjusted using the U.S. 1940 standard population with rates that were age-adjusted using the U.S. 2000 population.

Unless otherwise noted, all age-adjusted rates in NJSHAD have been adjusted using the US 2000 population standard.

Age-adjusted rates should be viewed as relative indexes, and used for comparison of populations. They are not actual measures of mortality risk, and do not convey the magnitude of the problem.

Table 1. U.S. 2000 Standard Population Weights for Age Standardization

	Age Group	U.S. 2000 Population Projection (in thousands)	Weight
1	Under 1 Year	3,795	0.013818
2	1 - 4 Years	15,192	0.055317
3	5 - 14 Years	39,977	0.145565
4	15 - 24 Years	38,077	0.138646
5	25 - 34 Years	37,233	0.135573
6	35 - 44 Years	44,659	0.162613
7	45 - 54 Years	37,030	0.134834
8	55 - 64 Years	23,962	0.087247
9	65 - 74 Years	18,136	0.066037
10	75 - 84 Years	12,315	0.044842
11	85 Years and Over	4,259	0.015508

Calculating Age-Adjusted Rates Using the Direct Method

To apply direct age-adjustment to a set of rates, the age-specific rate for each age group in the study population is multiplied by the appropriate weight in the standard population. The sum of these products is the directly age-adjusted, or age-standardized rate. The age-adjusted rate can be considered an average of each of the individual age-specific rates, but rather than being a simple average, it is a weighted average with each age-specific rate weighted by the proportion of people in the same age group in the standard population.

Tables 2a. and 2b. demonstrate the method used by NJSHAD in calculating age-adjusted rates. Notice that using crude death rates in Tables 2a. and 2b., one might conclude that whites have a higher underlying risk for heart disease death compared with blacks. How should the age-adjusted death rates be interpreted? You could use confidence intervals to assist in interpreting these data (NJSHAD automatically provides 95% confidence intervals for all rates).

Table 2a. Age-Adjusted Death Rate due to Heart Disease among Whites, New Jersey, 2004

Age Group	Number of Deaths (3-Year Sum)	Population Counts (3-Year Sum) [1]	Age- Specific Rate [2]	U.S. 2000 Standard Population Weight	Cross Products [3]
Under 5 years	4	307,139	1.30	0.069135	0.090037
5 - 14 years	3	709,758	0.42	0.145565	0.061527
15 - 24 years	15	634,256	2.36	0.138646	0.327894
25 - 34 years	35	576,010	6.08	0.135573	0.823780
35 - 44 years	169	883,005	19.14	0.162613	3.112281
45 - 54 years	542	886,502	61.14	0.134834	8.243639
55 - 64 years	1,126	647,215	173.98	0.087247	15.178901
65 - 74 years	2,109	411,158	512.94	0.066037	33.873118
75 - 84 years	5,679	353,102	1,608.32	0.044842	72.120157
85 yrs & over	7,621	141,128	5,400.06	0.015508	83.744167
All Ages	17,305	5,549,273	311.8 [a]	1	217.6 [b]

Table 2b. Age-Adjusted Death Rate due to Heart Disease among Blacks, New Jersey, 2004

Age Group	Number of Deaths (3-Year Sum)	Population Counts (3-Year Sum) [1]	Age- Specific Rate [2]	U.S. 2000 Standard Population Weight	Cross Products [3]
Under 5 years	3	91,279	3.29	0.069135	0.227221
5 - 14 years	2	188,782	1.06	0.145565	0.154215
15 - 24 years	8	178,727	4.48	0.138646	0.620593
25 - 34 years	24	159,695	15.03	0.135573	2.037479
35 - 44 years	84	186,190	45.12	0.162613	7.336319
45 - 54 years	205	149,037	137.55	0.134834	18.546381
55 - 64 years	309	98,175	314.74	0.087247	27.460477
65 - 74 years	470	58,583	802.28	0.066037	52.980199
75 - 84 years	609	32,097	1,897.37	0.044842	85.082026
85 yrs & over	3	1,584	189.45	0.015508	2.938097
All Ages	2,152	1,153,392	186.6 [a]	1	256.5 [b]

[1] New Jersey Department of Labor and Workforce Development, State Data Center

[2] Rate per 100,000 = (Age-specific death count * 100,000) / Age-specific population count

[3] Age-specific death rate * US2000 Std Pop Weight

[a] Crude death rate

[b] Age-adjusted rate.

Age adjustment is not appropriate if the age-specific death rates in the population of interest do not have a consistent relationship. For example, if the death rate among younger persons is increasing over time, but the death rate among older persons is decreasing over time, one would not want to age-adjust rates across years. One's conclusion of the trend in this death rate would be different, depending on which standard population is used. A younger standard population (such as the US 1940) would show an increase, whereas an older standard population (such as the US 2000) would show a decrease, or no change at all. Care should be taken so that the selection of the standard population does not affect the comparisons. For more information, see Curtin & Klein.

When reporting age-adjusted rates, always report the standard population used, and when comparing results to other data, be sure to document that those data were also age-adjusted and report the standard population. The age-adjusted rate is hypothetical, and is useful only for comparing populations, either over time, by geographic area, by sex or by racial/ethnic subgroups.

Although age-adjustment may be used with broad population age groups, such as adults (e.g., age 18+), it is not necessary (or meaningful) to age-adjust data for smaller age groups (e.g., age 18-24).

FAQs for Age-Adjustment:

Event Rates for a Subpopulation

Q:I am looking at death rates for female breast cancer. Which standard population should I use, females in US 2000 or all persons?
A:Theoretically, it doesn't matter, as long as you use the same standard population for all your analyses. But the recommended standard population is now the U.S. 2000 total population, even for analyses that apply only to a particular sex, race, or other subgroup.

When NOT to Age-Adjust

Q:Are there times I should NOT age-adjust?
A:Yes. Do NOT use age-adjustment when...

You are comparing populations with similar age distributions, and age-adjustment does not produce a rate that is substantively different from the crude rate.
You are comparing groups with the same, narrow, age range.
Do not use Direct Age-Adjustment if you have too few cases (you should have a least 20 events across all age groups). Instead, use Indirect Age-Adjustment.

Age Subpopulations

Q: I am looking at adults, only. If I use the weights in Table 1, above, they will not sum to one. Is that okay?
A: No. The weights must always sum to one. Weights for certain age subgroups have been published by the CDC. But you may also recompute the proportions in Table 1, using only the age range that is relevant to your analysis.

Age/Sex Adjusted Rates

Q: I have a report that uses age AND SEX adjusted rates. What is this, and why doesn't NJSHAD produce age and sex adjusted rates?
A: It is sometimes appropriate to adjust by other variables besides age. Rates that have been adjusted by age and sex use age- and sex-specific rates, weighted by twice the number of weights (one set for males and one set for females), but the total of all the weights still must sum to 1.0. NJSHAD doesn't compute these rates because there is little demand for it.

Confidence Intervals for Age-Adjusted Rates

Q: Can I use the confidence interval for the crude rate with the age-adjusted rate?
A: No, a new confidence interval for the age-adjusted rate must be calculated. Methods for calculation of this confidence interval may be found in Curtin & Klein.

Indirect Age-adjustment

In some cases, such as when there are too few cases to stratify by age, "indirect age standardization" may be used. Indirect standardization is based on standard mortality and morbidity ratios (SMR), and adjusts the age-specific rates found in the standard population to the age distribution of the smaller area or sub-population. According to Curtin & Klein, "One of the problems with [direct age adjustment] is that rates based on small numbers of deaths will exhibit a large amount of random variation. A very rough guideline is that there should be at least 25 total deaths over all age groups." NJSHAD follows NCHS's guideline of 20 total deaths. When fewer than 20 health events occurred over a time period, you may consider combining years, or using indirect age-adjustment.

The direct method can present problems when population sizes are particularly small. Calculating directly standardized rates requires calculating age-specific rates, and for small areas these age-specific rates may be based on only one or two events. In such cases, indirect standardization of rates may be used.

Indirectly standardized rates are based on the standard mortality or morbidity ratio (SMR) and the crude rate for a standard population. Indirect standardization adjusts the overall standard population death rate to the age distribution of the small area (Lilienfeld & Stolley). It is technically appropriate to compare indirectly standardized rates only with the rate in the standard population, not with each other.

Deciding Which Measure to Use

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

1. 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.

2. Klein RJ, Schoenborn CA. Age-Adjustment Using the 2000 Projected U.S. Population. Healthy People Statistical Notes, no. 20. Hyattsville, Maryland: National Center for Health Statistics. January 2001.

3. Curtin, LR, Klein, RJ. Direct Standardization (Age-Adjusted Death Rates). Healthy People Statistical Notes, no. 6. Hyattsville, Maryland: National Center for Health Statistics. March 1995.

4. Fleiss, JL. Statistical methods for rates and proportions. John Wiley and Sons, New York, 1973. As cited in Curtin and Klein, 1995.

5. Lilienfeld, DE and Stolley, PD. Foundations of Epidemiology, 3rd Ed. Oxford University Press, 1994.

New Jersey State Health Assessment Data New Jersey's Public Health Data Resource

Age-Adjusted Rates