Bias magnification and inflation. The ecologic bias (b
) equals the individual-level bias (b
) – due to confounding by group and effect modification of the risk difference by group – multiplied by the magnification factor M, assuming no other sources of ecologic bias. The ecologic bias also equals the sum of the individual-level bias due to ignoring groups and the bias caused by aggregation. The latter is measured by F, the inflation factor, equal to M - 1.