[R] heteroscedasticity problem
robert.ptacnik at niva.no
robert.ptacnik at niva.no
Wed Feb 7 21:52:06 CET 2007
Dear Listers,
I have a regression problem (x->y) with biological data, where x influences
y in two ways, (1) y increases with x and (2) the variation around the mean
(residuals) decreases with increasing x, i.e. y becomes more 'predictable'
as x increases.
The relationship is saturating, y~a + bx + cx^2, gives a very good fit.
I know basically how to test for heteroscedasticity. My question is if
there is an elegant regression method, which captures both, the mean and
the (non-constant) variation around the mean. Such a method would ideally
yield an estimate of the mean and its variation, both as a function of x.
The pattern corresponds very well to some established ecological theory
(each x is the species richness of a community of primary producers, y is
the productivity of each community; productivity and its predictability
both increase with increasing species richness).
Apologies for the probably clumsy decription of my problem - I am
ecologist, not statistician (but a big fan of R).
Cheers,
Robert
Robert Ptacnik
Norwegian Institute for Water Research (NIVA)
Gaustadalléen 21
NO-0349 Oslo
FON +47 982 277 81
FAX +47 221 852 00
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