|
Effect modification of the risk difference by group |
|||||||||
| 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 |
||||||
| Xi |
0.5 |
0.4 |
|||||||
| Yi |
0.15 |
0.3 |
|||||||
| ni |
200 |
200 |
|||||||
|
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, (Y1-Y0)/(X1-X0)= -1.5, is extremely biased. 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 |
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