Age Adjusted Rates

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Age Adjusted Rates
R.M. Pandey Additional Professor Department of
Biostatistics All India Institute of Medical
Sciences New Delhi
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• Crude Rates
• a single number computed as a summary
  measure for an entire group
• disregards differences caused by age, sex and
  other characteristics
• Adjusted Rates
• a single number obtained by adjusting for
  imbalance in age, sex and other characteristics
  while comparing a rate for two or more groups

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Example Hearing Impairment Due to Injury in 17
persons
Employment status Population Impairment Rate per 1,000
Currently employed 98,917 552 5.58
Currently unemployed 7,462 27 3.62
Not in the labor force 56,778 368 6.48
Total 163,157 947 5.80
Conclusion compare the crude rates between the
groups ? Is the conclusion valid ?
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The groups may differ substantially with respect
to age, sex and other important
characteristics e.g. two populations from
different geographical areas (A)
composed of entirely males (B) entirely females
- Can never be sure whether the
difference in mortality is due to location
or due to gender. Such a situation, gender is
referred as a confounder, because it associated
with with both location and death rate. It
obscures the relationship between location and
death rate.
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Compare age structure of the two populations
Age Currently Employed Currently Employed Not in labor Force Not in labor Force
Age Population Population
17-44 67,987 68.7 20,760 36.6
45-64 27,592 27.9 15,108 26.6
65 3,338 3.4 20,910 36.8
Total 98,917 100.0 56,778 100.0
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Age-specific impairment rates
Age Population Impairments Rate per 1,000
17-44 94,930 441 4.65
45-64 43,857 308 7.02
65 24,370 198 8.12
Total 163,157 947 5.80
5.80 is the weighted average of the age specific
rates
Age is a confounder in the relationship between
hearing impairment and employment status
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More accurate comparison age-specific
comparisons
Age Currently Employed Currently Employed Currently Employed Not in Labor Force Not in Labor Force
Age Popn. Impaire-ments Rate/1000 Popn. Impaire- ments
17-44 67,987 346 5.09 20,760 80
45-64 27,592 179 6.49 15,108 117
65 3,338 27 8.09 20,910 171
Total 98,917 552 5.58 56,778 368

Rate/ 1000
3.85
7.74
8.18
6.48

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More accurate comparison age-specific
comparisons
Age Currently Employed Currently Employed Currently Employed Not in Labor Force Not in Labor Force
Age Popn. Impaire-ments Rate/1000 Popn. Impair- Ements
17-44 67,987 346 5.09 20,760 80
45-64 27,592 179 6.49 15,108 117
65 3,338 27 8.09 20,910 171
Total 98,917 552 5.58 56,778 368

Rate/ 1000
3.85
7.74
8.18
6.48
Crude rate provided an incomplete picture of the
true situation

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• Subgroup specific rates
• More accurate comparison than crude rates
• We would end up with overwhelming number of
  rates to compare
• More Convenient
• Summarize an entire situation with a single
  number calculated for each subpopulation, a
  number that adjusts for difference in composition
• Two Ways
• Direct method of standardization
• Indirect method of standardization

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Direct Method of Standardization Step 1
Select the standard population. Step 2 compute
the expected events that would result if ,
instead of having different age distributions,
all populations were to have same standard age
structure Step 3 Compute the adjusted rate as
total expected events in the group divided by
the total standard population
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Total Total Currently Employed Currently Employed Not in Labor Force Not in Labor Force
Age (1) (2) (3) (4) (5)
Age StandardPopn. Rate/1000 Expected Impairm. Rate/1000 Expected Impairm.
17-44 94,930 5.09 483.2 3.85 365.5
45-64 43,857 6.49 284.6 7.74 339.5
65 24,370 8.09 197.2 8.18 199.3
Total 163,157 965.0 904.3
Age adjusted impairment rate for Currently
employed 965.0/163,157 5.91 per 1,000 Not
in the labor force 904.3/163,157 5.54 per
1,000
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Indirect Method of Standardization
Step 1 use a set of standard age-specific rates
along with the actual age composition of each
population Step 2 compute the number of events
that would have occurred in the two groups if
each took on the age specific rates of the
standard population while retaining its own age
distribution Step 3 compute standardized event
ratio as observed/expected events for each
group The indirect method often concludes with
this ratio. Step 4 actual age adjusted rates
for each group event rate in the standard
population x standardized event ratio of
the group
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Total Total Currently Employed Currently Employed Not in Labor Force Not in Labor Force
Age (1) (2) (3) (4) (5)
Age Rate/ 1,000 Population Expected Impairm. Population Expected Impairm.
17-44 4.65 67,987 316.1 20,760 96.5
45-64 7.02 27,592 193.7 15,108 106.1
65 8.12 3,338 27.1 20,910 169.8
Total 5.80 98,917 536.9 56,778 372.4
Standardized event rate Currently employed
552/536.9 1.03 103 Not in the
labor force 368/372.4 0.99 99
Age adjusted rates 5.08 x 1.03 5.97
5.08 x 0.99 5.74
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Uses of Standardised Rates
Smoking Group Death rate per 1,000 person years Death rate per 1,000 person years Death rate per 1,000 person years
Smoking Group A B C
Non smokers 20.2 11.3 13.5
Cigarettes 20.5 14.1 13.5
Cigar/pipes 35.5 20.7 17.4
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Smoking Group Mean age(years) Mean age(years) Mean age(years)
Smoking Group A B C
Non smokers 54.9 49.1 57.0
Cigarettes 50.5 49.8 53.2
Cigar/pipes 65.9 55.7 59.7
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Taking age group of non-smokers as standard
Smoking Group Death rate per 1,000 person years Death rate per 1,000 person years Death rate per 1,000 person years
Smoking Group A B C
Non smokers 20.2 11.3 13.5
Cigarettes 28.3 12.8 17.7
Cigar/pipes 21.2 12.0 14.2
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• Must know when to use an adjusted rate rather
  than crude rate
• If no confounders the crude rate is adequate
• If confounders present subgroup specific rates
  are sufficient
• Adjusted rates should be considered if they are
  meaningful
• If distribution of standard population is
  radically different than the populations being
  compared, standardization is inappropriate
• Also, when direct standardization is applied,
  subgroup specific rates should have same general
  trends in all the groups being compared as well
  as in the standard population
• Direct method of standardization is used more
  frequently than indirect method
• Direct method requires subgroup specific rates
  for all popns.
• Application of either method should lead to same
  conclusion

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