AgeAdjusted Rates
An
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 agespecific 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. Ageadjustment 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 ageadjusted rates may be accomplished using direct, or indirect age standardization.
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Direct ageadjustment (or age standardization) is the same as calculating a weighted average. It weights the agespecific 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 ageadjustment (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.
Unless otherwise noted, all ageadjusted rates in NJSHAD have been adjusted using the US 2000 population standard.
Ageadjusted rates should be viewed as
Table 1. U.S. 2000 Standard Population Weights for Age Standardization
Age Group 
U.S. 2000 Population Projection 
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 AgeAdjusted Rates Using the Direct Method
To apply direct ageadjustment to a set of rates, the agespecific 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 ageadjusted, or agestandardized rate. The ageadjusted rate can be considered an average of each of the individual agespecific rates, but rather than being a simple average, it is a weighted average with each agespecific 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 ageadjusted 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 ageadjusted 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. AgeAdjusted Death Rate due to Heart Disease among Whites, New Jersey, 2004
Age Group 
Number of Deaths 
Population Counts 
Age Specific Rate [2] 
U.S. 2000 Standard 
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 
Table 2b. AgeAdjusted Death Rate due to Heart Disease among Blacks, New Jersey, 2004
Age Group 
Number of Deaths 
Population Counts 
Age Specific Rate [2] 
U.S. 2000 Standard 
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 
[1] New Jersey Department of Labor and Workforce Development, State Data Center 
[2] Rate per 100,000 = (Agespecific death count * 100,000) / Agespecific population count 
[3] Agespecific death rate * US2000 Std Pop Weight 
[a] Crude death rate 
[b] 
Age adjustment is not appropriate if the agespecific 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 ageadjust 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 ageadjusted rates, always report the standard population used, and when comparing results to other data, be sure to document that those data were also ageadjusted and report the standard population. The ageadjusted rate is hypothetical, and is useful only for
FAQs for AgeAdjustment:
Event Rates for a Subpopulation
When NOT to AgeAdjust
 You are comparing populations with similar age distributions, and ageadjustment 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 AgeAdjustment if you have too few cases (you should have a least 20 events across all age groups). Instead, use Indirect AgeAdjustment.
Age Subpopulations
Age/Sex Adjusted Rates
Confidence Intervals for AgeAdjusted Rates
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 agespecific rates found in the standard population to the age distribution of the smaller area or subpopulation. 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 ageadjustment.
The direct method can present problems when population sizes are particularly small. Calculating directly standardized rates requires calculating agespecific rates, and for small areas these agespecific 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.
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?  Ageadjusted 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. AgeAdjustment 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 (AgeAdjusted 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.