Exact expressions for the weights used in least-squares regression   estimation for the log-logistic and Weibull distribution
  Estimation for the log-logistic and Weibull distributions can be performed by using the equations used for probability plotting. The equations leads to highly heteroscedastic regression. Exact expressions for the variances of the residuals are derived which can be used to perform weighted regression. In large samples maximum likelihood performs best, but it is shown that in smaller samples the weighted regression outperforms maximum likelihood estimation with respect to bias and mean square error. 
J.M. van Zyl
math.ST stat.TH