Ecologic studies JF Boivin S:\BOIVIN\695\Winter 2006\Ecologic studies (abbreviated).ppt (August 28, 2018)

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

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)

(Goodman DC, et al. NEJM 2002)

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

Ecologic study

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

Person-years Group 1 Group 2 E+ E- M1+ PY1T D+ PY ? PY1+ PY1- E+ E-

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

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

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

Ecologic fallacy

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

No ecologic bias RATE (per 100,000) % EXPOSURE 200 190 180 170 160 150 140 130 120 110 10 20 30 40 50 60 70 80 90 100 % EXPOSURE

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

Ecologic bias RATE (per 100,000) % EXPOSURE 200 190 180 170 160 150 140 130 120 110 10 20 30 40 50 60 70 80 90 100 % EXPOSURE

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

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

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

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

Aggregate exposure Attributes of individuals that are summarized at the group level Scientific interest may lie in: Individual effect Contextual effect

Intrinsically population-level exposure Attributes of groups for which no distinct analog exists at the individual level Everybody is exposed!

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!

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