1 Ecologic studies JF Boivin S:\BOIVIN\695\Winter 2006\Ecologic studies.ppt (December 5, 2015)

2 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies Reference rate varies 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

3 Percentage of children receiving measles-mumps- rubella immunization in second year of life and caseload of children with autism, by year of birth, California (Dales et al., JAMA 2001)

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5 (Goodman DC, et al. NEJM 2002)

6 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies Reference rate varies 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

7 Ecologic study A study in which the units of analysis are populations or groups of people, rather than individuals. (Last. 2001)

8 Structure of an ecologic study: Counts E+E- M 1+ M 1- N 1- N 1+ D+ D- ? ? ? ? Group 1 E+E- M 2+ M 2- N 2- N 2+ D+ D- ? ? ? ? Group 2

9 Person-years E+E- M 1+ PY 1T D+ PY ? PY 1+ ? PY 1- Group 1 E+E- M 2+ PY 2T D+ PY ? PY 2+ ? PY 2- Group 2

10 Durkheim’s study ProtestantOther 10 1,000,000 Suicide PY ? 300,000 ? 700,000 Group 1 (provinces with protestant minority) ProtestantOther 20 1,000,000 Suicide PY ? 800,000 ? 200,000 Group 2 (provinces with protestant majority)

11 Actual study ?? Group 1 Group 2 ???? Group 3 ?? Group 4

12 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies Reference rate varies across 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

13 Ecologic fallacy “… the mistaken assumption that a statistical association observed between two ecologic (group-level) variables… is equal to the association between the corresponding variables at the individual level…” (Encyclopedia of epidemiologic methods. 2000)

14 Ecologic fallacy “…the ecologic fallacy is due to cross-level bias in estimating the biologic effect of an exposure on disease risk on the basis of ecologic data… In an ecologic analysis involving simple linear regression, cross-level bias arises when the disease rate in the unexposed (reference) population is correlated with exposure prevalence across groups or when the difference in rates between exposed and unexposed populations (biologic effect) varies across groups.” (Encyclopedia of epidemiologic methods. 2000)

15 No ecologic bias E+E ,000 D+ PY 24 12, ,000 Group 2 (Ontario) IEIE = 200/100,000 IoIo =100/100,000 RD=100/100,000 RR=2 Group rate= 32/20,000 = 160/100,000 % exposure=12,000/20,000=60% Adapted from Rothman-Greenland Table 23-2 E+ E ,000 D+ PY 16 8, ,000 Group 1 (Québec) IEIE = 200/100,000 IoIo =100/100,000 RD RR =100/100,000 =2 Group rate= 28/20,000 = 140/100,000 % exposure=8,000/20,000=40%

16 No ecologic bias RATE (per 100,000) % EXPOSURE IRR==2 = IEIE IoIo 100/100, /100,000

17 Ecologic bias (rate difference varies across groups) E+E ,000 D+ PY 20 13, ,000 Group 2 (Ontario) IEIE = 154/100,000 IoIo =100/100,000 RD=54/100,000 RR=1.54 Group rate= 27/20,000 = 135/100,000 % exposure=13,000/20,000=65% E+ E ,000 D+ PY 20 7, ,000 Group 1 (Québec) IEIE = 286/100,000 IoIo =100/100,000 RD=186/100,000 RR=2.86 Group rate= 33/20,000 = 165/100,000 % exposure=7,000/20,000=35%

18 Ecologic bias RATE (per 100,000) % EXPOSURE IRR==0.5 = IEIE IoIo 200/100, /100,000

19 Ecologic bias (reference rate varies across groups) E+E ,000 D+ PY 40 16, ,000 Group 2 (Ontario) IEIE = 250/100,000 IoIo =150/100,000 RD=100/100,000 RR=1.67 Group rate= 46/20,000 = 230/100,000 % exposure=16,000/20,000=80% E+ E ,000 D+ PY 16 8, ,000 Group 1 (Québec) IEIE = 200/100,000 IoIo =100/100,000 RD=100/100,000 RR=2 Group rate= 28/20,000 = 140/100,000 % exposure=8,000/20,000=40%

20 Ecologic bias RATE (per 100,000) % EXPOSURE IRR==5.5 = IEIE IoIo 50/100, /100,000

21 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies Reference rate varies 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

22 Ecologic confounder

23 Ecologic confounder

24 Ecologic confounder

25 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies Reference rate varies 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

26 Aggregate exposure Attributes of individuals that are summarized at the group level Proportion of smokers Median family income Proportion of protestants Prevalence of subjects who are immune to measles Scientific interest may lie in: Individual effect Contextual effect

27 Attributes of groups for which no distinct analog exists at the individual level Population density Law Health-care system Social disorganization Income discrepancy Everybody is exposed! Intrinsically population-level exposure

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29 Neighborhood social class as aggregate of individual social classes Can differ from study subjects’ social class Neighborhood social class as contextual variable Same contextual variable for all subjects The variable is ecological, but the study is not!

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31 Outline 1.Examples 2.Definition 3.Ecologic fallacy Definition Valid ecologic study Rate difference varies across groups Reference rate varies across groups 4.Ecologic confounder 5.Types of ecologic exposures 6.Rationale for ecologic studies

32 1.Low cost and convenience 2.Measurement limitation of individual-level studies 3.Design limitations of individual- level studies 4.Simplicity of analysis and presentation ?Interest in ecologic effects

33 Dales L, et al. Time trends in autism and in MRR immunization coverage in California. JAMA 2001; 285: Durkheim E. Suicide: a study in sociology. London: Routledge & Kegan Paul Ltd, Gail MH, Benichou J, eds. Encyclopedia of epidemiology methods. New York: John Wiley & Sons, Ltd Goodman DC, et al. The relation between the availability of neonatal intensive care and neonatal mortality. NEJM 2002; 346: Last JM. A dictionary of epidemiology. Fourth edition. New York: Oxford University Press Rothman KJ, Greenland S. Modern epidemiology. Second edition. Philadelphia: Lippincott-Raven References