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Table 3 Effect modification of the risk difference by group

From: Bias magnification in ecologic studies: a methodological investigation

 

Group 0

Group 1

Crude

 

expose

unexpose

sum

expose

unexpose

sum

expose

unexpose

sum

case

20

10

30

48

12

60

68

22

90

noncase

80

90

170

32

108

140

112

198

310

total

100

100

200

80

120

200

180

220

400

risk

0.2

0.1

 

0.6

0.1

 

0.378

0.1

 

RD

0.1

  

0.5

  

0.278

  

X i

0.5

  

0.4

     

Y i

0.15

  

0.3

     

n i

200

  

200

     
  1. The background risks in each group are the same but the risk differences vary. The crude individual-level risk difference lies between the two extremes, as it must when exposure is binary. The ecologic estimate of the risk difference, (Y 1-Y 0)/(X 1-X 0)= -1.5, is extremely biased. 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