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Ecologic studies

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Ecologic studies

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  1. Ecologic studies JF Boivin Version 8 October 2010

  2. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

  3. Figure 1. Statistical (empirical Bayes) estimates of county-specific adjusted annual suicide rates in the United States. A Bayes estimate of 1.0 indicates that the rate for the county was equal to the national rate of 12 per 100 000 population, a Bayes estimate of 2.0 represents a doubling of the national rate, and a Bayes estimate of 0.5 represents half the national rate. The estimates are based on all data from 1996 to 1998, adjusted for age, sex, and race.

  4. Example: Time-trend studies Time Trends in Autism and in MMR Immunization Coverage in California Percentage of Children Receiving Measles-Mumps-Rubella (MMR) Immunization in Second Year of Life and Caseload of Children With Autism, by Year of Birth, California, 1980-1994 JAMA. 2001;285:1183-1185

  5. Émile Durkheim • Émile Durkheim (1858 –1917) was a famous French sociologist and pioneer in the development of modern sociology and anthropology. • In a groundbreaking book published in 1897, entitled Le Suicide, Durkheim explored the differing suicide rates among Protestants and Catholics. • In 19th century Europe, suicide rates were higher in countries that were more heavily Protestant. Durkheim found that suicide rates were highest in provinces that were heavily Protestant. • He concluded that stronger social control among Catholics resulted in lower suicide rates. • However, Durkheim's study of suicide was criticized as an example of the logical error termed the "ecological fallacy."

  6. Using ordinary least-squares linear regression on Durkheim's data, Morgenstern (1995) found a strong positive correlation (Figure below) between proportion protestant and suicide rates. The estimated rate ratio, comparing Protestants with other religions, was 7.6 (i.e. suicide rates among protestants was about 8 fold higher than other religions).

  7. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

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

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

  11. County 1 Antidepressant Yes No Suicide ? ? 10 PY 100,000 900,000 1,000,000 County i Suicide ? ? 20 PY 200,000 800,000 1,000,000 Gibbons’ study

  12. Outline • Examples • Definition • Ecologicfallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

  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- IE 28 D+ 16 12 = 100/100,000 Io 200/100,000 = 100/100,000 RD = PY 8,000 12,000 20,000 = 2 RR = = 40% % exposure = 100/100,000 RD = = Group rate 140/100,000 Io = 100/100,000 E+ E- IE 200/100,000 = 32 D+ 24 8 PY 12,000 8,000 20,000 = 2 RR = = 60% % exposure = = Group rate 160/100,000 No ecologic bias Group 1 (Québec) 8,000/20,000 28/20,000 Group 2 (Ontario) 12,000/20,000 32/20,000 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+ IE 286/100,000 = 33 D+ 20 13 Io = 100/100,000 PY 7,000 13,000 20,000 = 186/100,000 RD = 2.86 RR = = 35% % exposure = = Group rate 165/100,000 E+ E- IE 154/100,000 = 27 D+ 20 7 Io = 100/100,000 = 54/100,000 RD PY 13,000 7,000 20,000 = 1.54 RR = = 65% % exposure = = Group rate 135/100,000 Ecologic bias(rate difference varies across groups) Group 1 (Québec) 7,000/20,000 33/20,000 Group 2 (Ontario) 13,000/20,000 27/20,000

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

  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 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

  23. 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

  24. Example: ecologic effect

  25. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

  26. 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 5. Instrumental variables

  27. (Koepsell & Weiss)

  28. Outline • Examples • Definition • Ecologic fallacy • Definition • Valid ecologic study • Rate difference varies • Reference rate varies 4. Contextual effects • Rationale for ecologic studies • Ecologic confounders

  29. No ecologic bias

  30. Crude ecologic biasNo stratum-specific ecologic bias

  31. Ecologic bias (crude and stratum-specific)

  32. Another example