>Dear Dave,
Unfortunately the answer is negative in both cases.
>
>
>
> I wish to fit an error variance which varies smoothly with a time variable
e.g.
> a quadratic
> var(t) = b0 + b1 * t + b2 * t * t
> Is it possible to fit a function like this? I know I can divide my time
variable
> up into discrete intervals and get an error variance for each 'environment'
but
> a quadratic would fit fewer parameters.
>
What you propose has been suggested but not implemented as yet.
> A second question. When specifying an error which depends on the level of some
> environmental factor (with unequal class frequencies) is it necessary to
> specify, for the R structure, the numbers in each class and sort the data on
> this factor or is there a simple way of just specifying 'let the error vary
with
> the levels of this factor' ? Thanks.
There is no automatic way. You will have to sort the data
and specify the number in each group as separate sections with separate
variances.
Arthur
>
>
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Arthur Gilmour PhD mailto:Arthur.Gilmour@agric.nsw.gov.au
Principal Research Scientist (Biometrics) fax: <61> 2 6391 3899
NSW Agriculture <61> 2 6391 3922
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