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
X i	0.40			0.60
Y i	0.28			0.72
n i	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; X_i and Y_i are the average exposure and average risk in group i; n_i = size of group i.