spencer.graves at pdf.com
Mon Sep 4 21:30:44 CEST 2006
Have you considered talking logarithms of the expression you
log(Yield) = a1*log(A)+b1*log(B)+c2*log(C)+...
where a1 = a/(a+b+...), etc. This model has two constraints not present
in ordinary least squares: First, the intercept is assumed to be zero.
Second, the coefficients in this log formulation must sum to 1. If I
were you, I might use something like "lm" to test them both.
To explain how, I'll modify the notation, replacing A by X1, B by
X2, ..., up to Xkm1 (= X[k-1]) and Xk for k different environmental
variables. Then I might try something like the following:
fit0 <- lm(log(Yield) ~ log(X1) + ... + log(Xk)-1 )
fit1 <- lm(log(Yield) ~ log(X1) + ... + log(Xk) )
fit.1 <- lm(log(Yield/Xk) ~ log(X1/Xk) + ... + log(Xkm1/Xk) )
fit.0 <- lm(log(Yield/Xk) ~ log(X1/Xk) + ... + log(Xkm1/Xk)-1 )
anova(fit1, fit0) would test the no-constant model, and if I
haven't made a mistake in this, anova(fit0, fit.0) and anova(fit1,
fit.1) would test the constraint that all the coefficients should sum to
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Hope this helps.
Simone Vincenzi wrote:
> Dear R-list,
> I'm trying to estimate the relative importance of 6 environmental variables
> in determining clam yield. To estimate clam yield a previous work used the
> function Yield = (A^a*B^b*C^c...)^1/(a+b+c+...) where A,B,C... are the
> values of the environmental variables and the weights a,b,c... have not been
> calibrated on data but taken from literature. Now I'd like to estimate the
> weights a,b,c... by using a dataset with 110 observations of yield and
> values of the environmental variables. I'm wondering if it is feasible or if
> the number of observation is too low, if some data transformation is needed
> and which R function is the most appropriate to try to estimate the weights.
> Any help would be greatly appreciated.
> Simone Vincenzi
> Simone Vincenzi, PhD Student
> Department of Environmental Sciences
> University of Parma
> Parco Area delle Scienze, 33/A, 43100 Parma, Italy
> Phone: +39 0521 905696
> Fax: +39 0521 906611
> e.mail: svincenz at nemo.unipr.it
> R-help at stat.math.ethz.ch mailing list
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
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