STANDARDIZATION Direct Method Indirect Method

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). Standardization accounts for the differing distributions of the underlying characteristics

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).

AgePopul.# Deaths% of Popul.Death Rate 0 to 14190, % to 24115, % to 44289, % to 64180,3961, % ,4065, % TOTAL892,8748, % Hillsborough 1995 (top)Pinellas 1995 (bottom) AgePopul.# Deaths% of Popul.Death Rate 0 to 14138, % to 2483, % to 44239, % to 64190,4271, % ,57610, % TOTAL876,20012, %0.0142

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.

STANDARDIZATION H + PDeathRateExpectedDeaths AgePopul.HPHP 0 to 14329, to 24199, to 44528, , to 64370, ,9662, , ,52315,707 TOTAL1,769, ,09420,020 Adjusted Death Rate (H) = / 1,769,074 = 1,192 per 100K Adjusted Death Rate (P) = / 1,769,074 = 1,132 per 100K

STANDARDIZATION Adjusted Death Rate (H) = / 1,769,074 = 1,192 per 100K Adjusted Death Rate (P) = / 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.

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.

STANDARDIZATION U.S.DeathRateExpectedDeaths AgePopul.HPHP 0 to 1455,961, to 2436,124, to 4482,366, to 6448,345, ,283, Total255,079, Adjusted Death Rate (H) = Adjusted Death Rate (P) =

STANDARDIZATION U.S.DeathRateExpectedDeaths AgePopul.HPHP 0 to 1455,961, ,36444, to 2436,124, ,12428, to 4482,366, , to 6448,345, ,760362, ,283, ,568,9531,491,474 Total255,079, ,215,1692,100,696 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

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.

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).

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.

STANDARDIZATION Axioms (Direct Adjustment): 5.If you believe that one population is not exposed, and the other may be, choose the non-exposed group as the standard population. 6.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

STANDARDIZATION Reminders (Direct Adjustment): 1.Adjusted (standardized) rates are fictitious numbers. Their magnitude is related to the standard population chosen. 2.Adjusted rates should be used for comparison purposes only (i.e since they are hypothetical).

STANDARDIZATION Indirect Adjustment: 1.Conceptually similar to direct adjustment, but uses standard stratum-specific “rates” rather than standard stratum-specific “weights” (population counts). 2.Using the standard “rates,” we compare the the observed number of events to the expected number of events.

STANDARDIZATION Indirect Adjustment: THE RESULTS ARE PRESENTED AS THE “STANDARDIZED MORBIDITY” OR “MORTALITY” RATIO (SMR) observed cases (O) SMR = ( x 100) expected cases (E)

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. general population Exposure DeathsGen. PopExpected Age P-Y (Observed)Death Rate Deaths (100K P-Y) < 4512, , , , > 60 1, TOTAL38, SMR = O / E (x 100) = 36 / 34 = (x 100) = 105.9

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. general population Interpretation: SMR = We estimate that underground potash miners have a 6% higher risk of mortality than the general population.

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. general population Interpretation: SMR = But don’t 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).

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. copper miners Exposure DeathsCopper Miner Expected Age P-Y (Observed)Death Rate Deaths (100K P-Y) < 4512, , , , > 60 1, TOTAL38, SMR = O / E (x 100) =

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. copper miners Exposure DeathsCopper Miner Expected Age P-Y (Observed)Death Rate Deaths (100K P-Y) < 4512, , , , > 60 1, TOTAL38, SMR = O / E (x 100) = 36 / 26.2 = 1.374(100) = 137.4

EXAMPLE: INDIRECT ADJUSTMENT Underground potash miners vs. copper miners Interpretation: SMR = We estimate that underground potash miners have a 37% higher risk of mortality than copper miners.

STANDARDIZATION Axioms (Indirect Adjustment): 1.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 amongthe strata (i.e. person years of follow- up) will usually be different for each study population. Thus, the standard rates are not being applied tostandard weights.

STANDARDIZATION Axioms (Indirect Adjustment): 2.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