Table 2

Confounding by group

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

Xi
 0.40

0.60




Yi
 0.28

0.72




ni
 100

100




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, (Y1-Y0)/(X1-X0) = 2.2, is much more confounded. RD = risk difference; Xi and Yi are the average exposure and average risk in group i; ni = size of group i.

Webster Environmental Health 2007 6:17   doi:10.1186/1476-069X-6-17