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Table 2 Confounding by group

From: Bias magnification in ecologic studies: a methodological investigation

 

Group 0

Group 1

Crude

 

expose

unexposed

sum

expose

unexpose

sum

expose

unexpose

sum

case

16

12

28

48

24

72

64

36

100

noncase

24

48

72

12

16

28

36

64

100

total

40

60

100

60

40

100

100

100

200

risk

0.40

0.20

 

0.80

0.60

 

0.64

0.36

 

RD

0.20

  

0.20

  

0.28

  

X i

0.40

  

0.60

     

Y i

0.28

  

0.72

     

n i

100

  

100

     
  1. Since background risks and exposure distributions differ between the two groups, the crude individual-level estimate of the risk difference is confounded. The ecologic estimate of the risk difference, (Y 1-Y 0)/(X 1-X 0) = 2.2, is much more confounded. RD = risk difference; X i and Y i are the average exposure and average risk in group i; n i = size of group i.
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Environmental Health

ISSN: 1476-069X