Effect modification of the risk difference by group, illustrating Table 3. The solid black lines describing the individual-level information for the two groups have the same intercept (background risk q) but different slopes (risk differences b
0 ≠ b
1). The crude estimate of the risk difference b
(blue line) lies between these two extremes. Relative to b
, the ecologic estimate of the risk difference b
(red line) is far more biased than the crude individual-level estimate b
. Both biases are in the same direction. b
(purple line) is the weighted average of the risk differences used in the bias magnification equation.