[R] Question on WLS (gls vs lm)

From: Stats Wolf <stats.wolf_at_gmail.com>
Date: Thu, 24 Jun 2010 09:16:57 +0200


Hi all,

I understand that gls() uses generalized least squares, but I thought that maybe optimum weights from gls might be used as weights in lm (as shown below), but apparently this is not the case. See:

library(nlme)
f1 <- gls(Petal.Width ~ Species / Petal.Length, data = iris, weights = varIdent(form = ~ 1 | Species))
aa <- attributes(summary(f1)$modelStruct$varStruct)$weights f2 <- lm(Petal.Width ~ Species / Petal.Length, data = iris, weights = aa)

summary(f1)$tTable; summary(f2)

So, the two models with the very same weights do differ (in terms of standard errors). Could you please explain why? Are these different types of weights?

Many thanks in advance,
Stats Wolf



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