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Dear R-help members, I have a question regarding how to use varComb function to specify a variance function for the "weights" in the gls. I need to fit a linear model with heteroscedasticity. The variance function is exp(c0+nu0*W +nu1*W^2) where W is a covariate. Initially I want to use varFunc to define my own variance function following the instruction in the Pinheiro and Bates (2000), but I could not make it work. Then I used varComb in gls with weights=varComb(varExp(form=~W), varExp(form=~I(W^2))))). But the esti...

Dear R-help members, I have a question regarding how to use varComb function to specify a variance function for the "weights" in the gls. I need to fit a linear model with heteroscedasticity. The variance function is exp(c0+nu0*W +nu1*W^2) where W is a covariate. Initially I want to use varFunc to define my own variance function following the instruction in the Pinheiro and Bates (2000), but I could not make it work. Then I used varComb in gls with weights=varComb(varExp(form=~W), varExp(form=~I(W^2))))). But the esti...

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