The influence curve of robust fitting. This curve is the derivative of the curve shown in the right panel of Figure 13. The influence peaks for points whose distance from the curve equals the robust standard deviation of the residuals (RSDR). The RSDR is recomputed every iteration. This means that about two-thirds of the points get about the same influence they would have had with least-squares regression.
Motulsky and Brown BMC Bioinformatics 2006 7:123 doi:10.1186/1471-2105-7-123