Ecologic studies

1. Ecologic studies JF Boivin Version 27 November 2006 S:\BOIVIN\695\Winter 2007\Ecologic studies.ppt

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

5. (Goodman DC, et al. NEJM 2002)

6. (Goodman DC, et al. NEJM 2002)

7. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Ecologic confounder • Types of ecologic exposures • Rationale for ecologic studies

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

9. E+ E+ E- E- D+ ? ? M2+ M1+ D+ ? ? D- ? ? M1- D- ? ? M2- N1+ N2+ N2- N1- Structure of an ecologic study: Counts Group 1 Group 2

10. E+ E+ E- E- D+ D+ ? ? ? ? M1+ M2+ PY PY PY2+ PY1+ PY2- PY1- PY1T PY2T Person-years Group 1 Group 2

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

12. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies across • Ecologic confounder • Types of ecologic exposures • 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 “…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. E- E+ 200/100,000 = 28 D+ 16 12 = 100/100,000 RD = 100/100,000 RD = 100/100,000 PY 8,000 12,000 20,000 RR = 2 RR = 2 % exposure = 12,000/20,000 = 60% % exposure = 8,000/20,000 = 40% Io = 100/100,000 Group rate = = 160/100,000 32/20,000 Group rate = = 140/100,000 28/20,000 IE 200/100,000 = E+ E- 32 D+ 24 8 PY 12,000 8,000 20,000 No ecologic bias Group 1 (Québec) IE Io Group 2 (Ontario) Adapted from Rothman-Greenland Table 23-2

16. IE Io 200/100,000 IRR = = = 2 100/100,000 No ecologic bias 200 190 180 170 Ontario 160 150 RATE (per 100,000) Québec 140 130 120 110 10 20 30 40 50 60 70 90 100 80 % EXPOSURE

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

18. IE Io 100/100,000 IRR = = = 0.5 Ecologic bias 200 190 180 170 160 150 RATE (per 100,000) 140 130 120 110 10 20 30 40 50 60 70 90 100 80 % EXPOSURE 200/100,000

19. E- E+ 28 D+ 16 12 RD RD = = 100/100,000 100/100,000 PY 8,000 12,000 20,000 RR RR = = 2 1.67 % exposure % exposure = = 8,000/20,000 16,000/20,000 = = 80% 40% Io Io = = 150/100,000 100/100,000 Group rate Group rate = = = = 230/100,000 140/100,000 46/20,000 28/20,000 IE IE 200/100,000 250/100,000 = = E+ E- 46 D+ 40 6 PY 16,000 4,000 20,000 Ecologic bias(reference rate varies across groups) Group 1 (Québec) Group 2 (Ontario)

20. IE Io 275/100,000 IRR = = = 5.5 50/100,000 Ecologic bias 250 200 150 RATE (per 100,000) 100 0 10 20 30 40 50 60 70 90 100 80 % EXPOSURE

21. (Koepsell & Weiss)

22. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies • Ecologic confounder • Types of ecologic exposures • Rationale for ecologic studies

23. No ecologic bias

24. Crude ecologic biasNo stratum-specific ecologic bias

25. Ecologic bias (crude and stratum-specific)

26. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies • Ecologic confounder • Types of ecologic exposures • Rationale for ecologic studies

27. Aggregate exposure • 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 Attributes of individuals that are summarized at the group level

28. Intrinsically population-level exposure 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!

30. 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!

32. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies across groups • Reference rate varies across groups • Ecologic confounder • Types of ecologic exposures • Rationale for ecologic studies

33. Rationale for ecologic studies • Low cost and convenience • Measurement limitation of individual-level studies • Design limitations of individual-level studies • Koepsell and Weiss, Figure 12.1 • Simplicity of analysis and presentation ? Interest in ecologic effects

34. (Koepsell & Weiss)