EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

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STANDARDIZATION

Direct Method Indirect Method
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STANDARDIZATION
Issue Often times, we wish to compare
mortality rates between populations, or at
different time periods in one population,
however, the population groups may differ with
respect to underlying characteristics (e.g. age,
gender) that may affect the overall mortality
rate (hence not a fair comparison). Standardiz
ation accounts for the differing distributions of
the underlying characteristics
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STANDARDIZATION
Direct Standardization Adjusted rates are
derived by applying category-specific rates
observed in each population to a single standard
population. The adjusted rate represents the
hypothetical rate that would have been observed
had each population had the same distribution on
the confounding factor of concern (i.e. age).
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Hillsborough 1995 (top) Pinellas 1995 (bottom)
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STANDARDIZATION
Direct Standardization One way to select the
standard population is to combine population
counts from the populations. We can then apply
the category-specific death rates to the
standard population to calculate and compare the
expected number of deaths in each population.
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STANDARDIZATION
Adjusted Death Rate (H) 21094 / 1,769,074
1,192 per 100K Adjusted Death Rate (P) 20020 /
1,769,074 1,132 per 100K
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STANDARDIZATION
Adjusted Death Rate (H) 21094 / 1,769,074
1,192 per 100K Adjusted Death Rate (P) 20020 /
1,769,074 1,132 per 100K Age-adjusted rate
ratio 1,192 / 1,132 1.05 Using Hillsborough
Pinellas county as the standard
population The age-adjusted 1995 death rate
appears to be approximately 5 higher in
Hillsborough county compared to Pinellas county.
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STANDARDIZATION
Direct Standardization Another way to select
the standard population is to use an external
standard population, such as the United States
population. As before, we can then apply the
category-specific death rates to the standard
population to calculate and compare the expected
number of deaths in each population.
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STANDARDIZATION
Adjusted Death Rate (H) Adjusted Death Rate
(P)
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STANDARDIZATION
Adj. Death Rate (H) 2,215,169 / 255,079,000
868 per 100K Adj. Death Rate (P) 2,100,696 /
255,079,000 824 per 100K
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STANDARDIZATION
Adj. Death Rate (H) 2,215,169 / 255,079,000
868 per 100K Adj. Death Rate (P) 2,100,696 /
255,079,000 824 per 100K Age-adjusted rate
ratio 868 / 824 1.05 Using the United States
as the standard population The age-adjusted
1995 death rate appears to be approximately 5
higher in Hillsborough county compared to
Pinellas county.
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STANDARDIZATION
Axioms (Direct Adjustment) 1. If the
distribution of the population among the strata
is the same for the two populations
being compared, then adjustment is not
needed. 2. If the distribution of the population
among the strata is different for the two
populations, adjustment is indicated (regardless
of whether the stratum-specific rates are
similar or different).
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STANDARDIZATION
Axioms (Direct Adjustment) 3. If the
stratum-specific rates differ, calculating
an adjusted rate will mask important
differences. Thus, the adjusted rates should be
compared cautiously, and the stratum-specific
rates should be mentioned. 4. The selection of
the standard population is arbitrary. However,
when 2 or 3 populations are being compared, use
the sum of the populations.
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STANDARDIZATION

• Axioms (Direct Adjustment)
• If you believe that one population is not
  exposed,
• and the other may be, choose the non-exposed
• group as the standard population.
• You can choose a standard standard population
• They usually come in sets of three
• --- Developing World Standard weighted to the
  young
• --- Developed World Standard weighted to
  adults
• --- Global World Standard average of the above
  two

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STANDARDIZATION

• Reminders (Direct Adjustment)
• Adjusted (standardized) rates are fictitious
• numbers. Their magnitude is related to the
• standard population chosen.
• Adjusted rates should be used for comparison
• purposes only (i.e since they are hypothetical).

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STANDARDIZATION

• Indirect Adjustment
• Conceptually similar to direct adjustment, but
  uses standard stratum-specific rates rather
  than standard stratum-specific weights
  (population counts).
• Using the standard rates, we compare the
• the observed number of events to the expected
  number of events.

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STANDARDIZATION

• Indirect Adjustment
• THE RESULTS ARE PRESENTED AS THE
• STANDARDIZED MORBIDITY OR
• MORTALITY RATIO (SMR)
• observed cases (O)
• SMR ------------------------ ( x 100)
• expected cases (E)

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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. general population
Exposure Deaths Gen. Pop Expected Age
P-Y (Observed) Death Rate Deaths (100K
P-Y) ---------------------------------------------
----------------------------------- lt 45 12,140
1 50 6.1 45 - 49 8,863
4 79 7.0 50 - 54 10,095
11 105 10.6 55 - 59 5,924
14 120 7.1 gt 60 1,280 6
250 3.2 TOTAL 38,302 36
--- 34.0 -------------------------------
-------------------------------------------------
SMR O / E (x 100) 36 / 34 1.059 (x 100)
105.9
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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. general population
Interpretation SMR 105.9 We estimate that
underground potash miners have a 6 higher risk
of mortality than the general population.
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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. general population
Interpretation SMR 105.9 But dont forget
about the healthy worker effect (e.g. we expect
workers to have a lower mortality than the
general population). Also, beware that the
general population almost always contains some
exposed individuals (e.g. bias toward the null).
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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. copper miners
Exposure Deaths Copper Miner
Expected Age P-Y (Observed) Death Rate
Deaths (100K P-Y) ---------------------------
--------------------------------------------------
--- lt 45 12,140 1 35 45 - 49
8,863 4 64 50 - 54 10,095
11 89 55 - 59 5,924
14 87 gt 60 1,280 6
170 TOTAL 38,302 36 ---
-------------------------------------------
------------------------------------- SMR O /
E (x 100)
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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. copper miners
Exposure Deaths Copper Miner
Expected Age P-Y (Observed) Death Rate
Deaths (100K P-Y) ---------------------------
--------------------------------------------------
--- lt 45 12,140 1 35
4.2 45 - 49 8,863 4 64
5.7 50 - 54 10,095 11 89
9.0 55 - 59 5,924 14 87
5.2 gt 60 1,280 6 170
2.2 TOTAL 38,302 36 ---
26.2 ---------------------------------------------
----------------------------------- SMR O / E
(x 100) 36 / 26.2 1.374(100) 137.4
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EXAMPLE INDIRECT ADJUSTMENTUnderground potash
miners vs. copper miners
Interpretation SMR 137.4 We estimate that
underground potash miners have a 37 higher risk
of mortality than copper miners.
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STANDARDIZATION

• Axioms (Indirect Adjustment)
• It is usually not appropriate to compare a number
  of SMRs from different study populations with
  each other.
• This is because the distribution of exposure
  among the strata (i.e. person years of follow-up)
  will usually be different for each study
  population.
• Thus, the standard rates are not being applied
  to standard weights.

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STANDARDIZATION

• Axioms (Indirect Adjustment)
• Direct standardization is preferred over indirect
• standardization. Still, indirect
  standardization
• Has been traditionally used in public health
• Has intuitive appeal and is easy to understand
• Works well almost all of the time -- despite
  not
• guaranteeing standardization
• Is well known by journal editor and readers