In Eq. (2), ri are residuals, hi are leverage, S is mean absolute deviation divided by a factor 0.6745 to make it an unbiased estimator of standard deviation, and K is a tuning constant whose default value of 4.685 provides for 95% asymptotic efficiency as the ordinary least
squares assuming the Gaussian distribution (Holland and Welsch, 1977). Observations assigned zero weights in any iteration are regarded as outliers and removed from further computation.