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Ecologic studies. JF Boivin Version 27 November 2006. 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. - PowerPoint PPT Presentation
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S:\BOIVIN\695\Winter 2007\Ecologic studies.ppt
Ecologic studies
JF BoivinVersion 27 November 2006
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)
4
5
(Goodman DC, et al. NEJM 2002)
6
(Goodman DC, et al. NEJM 2002)
7
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
8
Ecologic study
A study in which the units of analysis are populations or groups of people, rather than individuals.
(Last. 2001)
9
Structure of an ecologic study: Counts
E+ E-
M1+
M1-
N1-N1+
D+
D-
?
?
?
?
Group 1
E+ E-
M2+
M2-
N2-N2+
D+
D-
?
?
?
?
Group 2
10
Person-years
E+ E-
M1+
PY1T
D+
PY
?
PY1+
?
PY1-
Group 1
E+ E-
M2+
PY2T
D+
PY
?
PY2+
?
PY2-
Group 2
11
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)
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
“…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-
32
20,000
D+
PY
24
12,000
8
8,000
Group 2 (Ontario)
IE = 200/100,000
Io = 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-
28
20,000
D+
PY
16
8,000
12
12,000
Group 1 (Québec)
IE = 200/100,000
Io = 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
110
120
140
130
150
160
170
180
190
200
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
IRR = = 2=IE
Io 100/100,000
200/100,000
Québec
Ontario
17
Ecologic bias(rate difference varies across groups)
E+ E-
27
20,000
D+
PY
20
13,000
7
7,000
Group 2 (Ontario)
IE = 154/100,000
Io = 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-
33
20,000
D+
PY
20
7,000
13
13,000
Group 1 (Québec)
IE = 286/100,000
Io = 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
110
120
140
130
150
160
170
180
190
200
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
IRR = = 0.5=IE
Io 200/100,000
100/100,000
19
Ecologic bias(reference rate varies across groups)
E+ E-
46
20,000
D+
PY
40
16,000
6
4,000
Group 2 (Ontario)
IE = 250/100,000
Io = 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-
28
20,000
D+
PY
16
8,000
12
12,000
Group 1 (Québec)
IE = 200/100,000
Io = 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
0
100
150
200
250
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
IRR = = 5.5=IE
Io
275/100,000
50/100,000
21
(Koepsell & Weiss)
22
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
23
No ecologic bias
Country 1 Country 2 Country 3
Covariate Exposure PY Rate PY Rate PY Rate
Yes Yes 3000 600 4000 600 4000 600
No 12000 500 8000 500 6000 500
RR 1.2 1.2 1.2
No Yes 2000 600 4000 600 6000 600
No 8000 500 8000 500 9000 500
RR 1.2 1.2 1.2
Sum Yes 5000 600 8000 600 10000 600
No 20000 500 16000 500 15000 500
RR 1.2 1.2 1.2
(rates per 100,000 person-year)(Note: no individual-level confounding)
Ecologic analysis
% exposed 20% 33% 40%
% covariate 60% 50% 40%
Overall disease rate 520 533 540
Crude RR = 1.2
Adjusted RR = 1.2
Crude is valid!
24
Country 1 Country 2 Country 3
Covariate Exposure PY Rate PY Rate PY Rate
Yes Yes 3000 600 4000 600 4000 600
No 12000 500 8000 500 6000 500
RR 1.2 1.2 1.2
No Yes 2000 200 4000 200 6000 200
No 8000 100 8000 100 9000 100
RR 2 2 2
Sum Yes 5000 440 8000 400 10000 360
No 20000 340 16000 300 15000 260
RR 1.3 1.3 1.4
(rates per 100,000 person-year)(Note: no individual-level confounding)
Ecologic analysis
% exposed 20% 33% 40%
% covariate 60% 50% 40%
Overall disease rate 360 333 300
Crude RR = 0.3
Adjusted RR = 1.3
Adjusted is valid!
Crude ecologic biasNo stratum-specific ecologic bias
25
Country 1 Country 2 Country 3
Covariate Exposure PY Rate PY Rate PY Rate
Yes Yes 8000 500 13000 1500 14000 1000
No 12000 500 12000 1500 6000 1000
RR 1 1 1
No Yes 2000 100 2000 300 6000 200
No 28000 100 23000 300 24000 200
RR 1 1 1
Sum Yes 10000 420 15000 1340 20000 760
No 40000 220 35000 711 30000 360
RR 1.9 1.9 2.1
(rates per 100,000 person-year)
Ecologic analysis
% exposed 20% 30% 40%
% covariate 40% 50% 40%
Overall disease rate 260 900 520
Crude RR = 8.6
Adjusted RR = 8.6
No valid estimate available!
Ecologic bias (crude and stratum-specific)
26
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
27
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
28
Attributes of groups for which no distinct analog exists at the individual level
Intrinsically population-level exposure
• Law • Health-care system• Social disorganization• Income discrepancy
Everybody is exposed!
• Population density
29
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!
31
32
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
33
1. Low cost and convenience
2. Measurement limitation of individual-level studies
3. Design limitations of individual-level studies
• Koepsell and Weiss, Figure 12.1
4. Simplicity of analysis and presentation
? Interest in ecologic effects
Rationale for ecologic studies
34
(Koepsell & Weiss)