On Fri, 27 Jul 2001, Nadine Wells wrote in part:
> Does anyone know what the power link function does in SAS? [...] when
> I plot the equation based on the parameter estimates, the model doesn't
> seem to look like I want it to. [...] I am trying to get SAS to run a
> model that resembles exponential growth. Any suggestions would be
> greatly appreciated.
Nadine, if you want to model exponential growth, why are you trying to
use a power function? For simple exponential growth,
Y = a e^(bT) where T is time (years, in your case?) and Y is
the variable whose growth is modelled. Then
log(Y) = log(a) + bT which is a simple linear regression.
It is easy to show that the doubling time is log(2)/b (all logs are
natural logs, of course). It then remains to adapt the model to include
your habitat complexity. How best to do this is not clear to me; but
at least you could start by breaking that variable into (probably
ordered?) categories, and try fitting a separate exponential function for
each such category (like an ANOVA), perhaps subject to one or more
constraints (e.g., a common value of b for all habitats -- that
analysis would resemble, formally, an analysis of covariance but you
might well prefer to model it in multiple regression terms).
Hope this helps, some.
--Don.
Nadine's complete post:
"Does anyone know what the power link function does in SAS? I have to
provide a parameter estimate in parenthases after the link=power command.
I've been using -1 but when I plot the equation based on the parameter
estimates, the model doesn't seem to look like I want it to. Does anyone
know exactly what the power link function does? More specifically, I am
trying to get SAS to run a model based on proportion data. That is, my
dependent variable is a proportion (# of beach seine hauls that catch
fish over total # of beach seines hauled), my explanatory variable is a
measure of habitat complexity. I am also using year as a categorical
variable. I am trying to get SAS to run a model that resembles
exponential growth. Any suggestions would be greatly appreciated.
Nadine"
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Donald F. Burrill [EMAIL PROTECTED]
184 Nashua Road, Bedford, NH 03110 603-471-7128
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