Post on 19-Jan-2016
description
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Ecologic studies
JF Boivin
S:\BOIVIN\695\Winter 2006\Ecologic studies (abbreviated).ppt (April 21, 2023)
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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
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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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(Goodman DC, et al. NEJM 2002)
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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
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Ecologic study
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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
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Person-years
E+ E-
M1+
PY1T
D+
PY
?
PY1+
?
PY1-
Group 1
E+ E-
M2+
PY2T
D+
PY
?
PY2+
?
PY2-
Group 2
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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)
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Actual study
? ?
Group 1
Group 2
? ?
? ?
Group 3
? ?
Group 4
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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
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Ecologic fallacy
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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
Io
RD
RR
Group rate =
% exposure =
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No ecologic bias
110
120
140
130
150
160
170
180
190
200
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
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Ecologic bias(rate difference varies across groups)
E+ E-
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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
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
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Ecologic bias
110
120
140
130
150
160
170
180
190
200
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
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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%
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Ecologic bias
0
100
150
200
250
100908070605040302010
RA
TE
(pe
r 10
0,00
0)
% EXPOSURE
IRR = = 5.5=IE
Io 50/100,000
275/100,000
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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
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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
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Aggregate exposure
Attributes of individuals that are summarized at the group level
Scientific interest may lie in:• Individual effect• Contextual effect
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Attributes of groups for which no distinct analog exists at the individual level
Everybody is exposed!
Intrinsically population-level exposure
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• 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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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