Figure 6.
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 b0 ≠ b1). The crude estimate of the risk difference bc (blue line) lies between these two extremes. Relative to bw, the ecologic estimate of the risk difference be (red line) is far more biased than the crude individual-level estimate bc. Both biases are in the same direction. bw (purple line) is the weighted average of the risk differences used in the bias magnification equation.