Epidemiology Kept Simple. Chapter 7. Rate Adjustment. (c) B. Gerstman 2007 Chapter 7: Age Adjustment 1

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1 Epidemiology Kept Simple Chapter 7 Rate Adjustment (c) B. Gerstman 2007 Chapter 7: Age Adjustment 1

2 Confounding Confounding a systematic error in inference due to extraneous factors Potential confounder an extraneous factor that can cause confounding Statistical techniques exist to adjust for confounding One such technique is rate adjustment (also called rate standardization) (c) B. Gerstman 2007 Chapter 7: Age Adjustment 2

3 Terminology For uniformity of language, let rate any incidence or prevalence, denoted R or r Crude rate (synonym: unadjusted rate) rate for entire population, denoted cr Strata-specific rate rate within a strata (subgroup), denoted r i, for rate within stratum i Adjusted rate (synonym: standardized rate) mathematically compensated rate, denoted ar (c) B. Gerstman 2007 Chapter 7: Age Adjustment 3

4 Mortality per 1000 by State, 1991 i Age Deaths Alaska Pop. ( 1000) Deaths Florida Pop. ( 1000) , ,113 3, ,400 4, ,108 2, ,977 1, ,483 1,038 TOTAL 2, ,258 13,278 Crude rate, Alaska Crude rate, Florida ,258 cralask. 3.9 cr Florida ,278 Chapter 7: Age Adjustment 4

5 Strata-Specific Rates by State Rates are per 1,000 p-yrs i Age Alaska Florida Age-specific rates are similar to each other! Why did Florida have a much higher crude rate? (c) B. Gerstman 2007 Chapter 7: Age Adjustment 5

6 Age Distributions Age AK % FL % % 915 7% % % % % % % % % >75 7 1% % TOTAL % %

7 Properties of Confounding Exposure associated with the confounder Confounder is an independent risk factor Confounder is not intermediary in the causal pathway [Age] Confounder Exposure Disease [State] [Mortality] (c) B. Gerstman 2007 Chapter 7: Age Adjustment 7

8 What can we do about confounding? Like-to-like (strata-specific) comparisons (e.g., 80-year olds to 80-year olds) Mathematical adjustments: Direct and indirect standardization (Ch 7) Other stratified techniques: Mantel- Haenszel techniques (Ch 14) Regression models (next edition) (c) B. Gerstman 2007 Chapter 7: Age Adjustment 8

9 7.2 Direct Adjustment N i N r wi r where N w i i N reference populationsize, strata i i ar direct reference population totalsize rate, study population, strata i ar direct is a weighted average of strata-specific rates i (c) B. Gerstman 2007 Chapter 7: Age Adjustment 9

10 Standard Million 1991 Reference Weight, strata i (w i ) = proportion in reference pop = N i / N i Age N i w i , , , , , , N = 1,000, (c) B. Gerstman 2007 Chapter 7: Age Adjustment 10

11 Alaska, Direct Adjustment (Rates are per 1000) Rate Weights Product i Age r i w i w i r i ar Alask w i r = wi ri (c) B. Gerstman 2007 Chapter 7: Age Adjustment 11

12 Florida, Direct Adjustment (Rates are per 1000) Rate Weights Product i r i w i w i r i ar Florida w i r = (c) B. Gerstman 2007 Chapter 7: Age Adjustment 12

13 Conclusions cr FL (10.3) > cr AK (3.9) ar FL (7.8) < ar AK (8.4) Age confounded the crude comparison State (E) associated with age (C) Age (C) is independent risk factor for death (D) Age (C) is not an intermediary in the causal pathway between E and D (c) B. Gerstman 2007 Chapter 7: Age Adjustment 13

14 7.3 Indirect Adjustment This method is based on a statistic called the Standardized Mortality Ratio (SMR): Observed SMR Expected where Observed observed no. Expected expected no. of of cases cases (c) B. Gerstman 2007 Chapter 7: Age Adjustment 14

15 Example: Zimbabwe & US Crude death rate in US (1991) = 880 per 100,000 Crude death rate in Zimbabwe = 886 per 100,000 But, US population much older (c) B. Gerstman 2007 Chapter 7: Age Adjustment 15

16 Zimbabwe Observed number of deaths, Zimbabwe = 98,808 Zimbabwe age distribution on p. 148 Use U.S. rates as reference rates (p. 149) Expected frequencies calculated on next page (c) B. Gerstman 2007 Chapter 7: Age Adjustment 16

17 Zimbabwe Expected frequencies calculated as follows: i Age US R i Zimbabwe n i μ i = R i n i ,899,204 4, ,537,992 3, ,386,079 4, ,235 7, ,387 5, ,109 10,951 Expected i ,951 36,381 (c) B. Gerstman 2007 Chapter 7: Age Adjustment 17

18 Zimbabwe SMR SMR Observed Expected 98,808 36, After adjusting for age, Zimbabwe s mortality rate is 2.72 times that of the U.S. (c) B. Gerstman 2007 Chapter 7: Age Adjustment 18

19 Indirect Adjustment (optional) ar indirect ( cr)( SMR) Zimbabwe crude rate = 886 (per 100,000) ar indirect = cr SMR = (886)(2.72) = 2410 Not necessary: interpret the SMR directly (instead) (c) B. Gerstman 2007 Chapter 7: Age Adjustment 19

20 7.4: Adjustment for Multiple Factors Same methods, just stratify by all factors For example, to control for age & gender stratify you could stratify as follows 0-4 females 0-4 males 5-9 females Yada yada yada then apply standardization method (c) B. Gerstman 2007 Chapter 7: Age Adjustment 20

21 Conclusion Crude rates cannot be used to compare populations without evaluating potential confounders such as age The simplest method of confounder adjustment is direct and indirect standardization (c) B. Gerstman 2007 Chapter 7: Age Adjustment 21

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