How can I set a system of equations to predict biomass using weighted restricted SUR in SAS?

02 February 2015 4 4K Report

Hi,

I´m using PROC SYSLIN in SAS program to set a system of equations to predict biomass in Acacia spp. My goal is to make a weighted restricted SUR fit (or WRSUR, like Parresol, 1999) as there is correlation between the errors of the equations. I have different equations to predict the biomass of stem, foliage, branches and total biomass, each with different functions of variance, for example:

Biomass(stem)=a0+a1*(DAC2*H)+ε

Biomass(branches)=b0+b1*(DAC2*H)+ε

Biomass(foliaje)=c0+c1*(DAC2)+ε

Biomass(total)=d0+d1*(DAC2)+d2*(DAC2*H)+ε

And the equations of the error variance, wich will be used as weights are:

σ2e(stem)=(DAC2*H)1.806,

σ2e (branches)=(DAC2)1.8744

σ2e (foliaje)=(DAC2*H)1.607

σ2e (total)=(DAC2*H)1.998

DAC=Diameter at the neck of the tree

H=Total tree height

I had no problems with a restricted SUR fit in SYSLIN, the procedure is relatively simple if the restrictions are well established through SRESTRICT. However, it seems that I have problems syntax to perform a WRSUR, since I do not know how to indicate the weights for the corresponding equation. I just know I should define the four weights in the input data set, but then not as assigning weights to each equation corresponding to conduct a weighted restricted SUR fit. Also, I have to define a variance-covariance previously in SYSLIN?. Can anyone help me with the correct syntax?, I appreciate a lot since I`m stuck.

B. Gurugnanam

No idea about this work.

Yahya Al-Satari

May be it is easy to do a regression relation between different treats and actual yield.

Hormoz Sohrabi

I asked this question once, you can check te answers. Most of the answers suggested using R. I used Parresol data as test data abd the results wre different. Finally, I Did the analyzes in statistica 12. This analyze is so user-friendly in this software.

https://www.researchgate.net/post/How_to_calculate_nonlinear_seemingly_unrelated_regression_NSUR

José Riofrío

you can try this:

proc model data=simul;

var Biomass(stem)Biomass(branches) Biomass(foliaje) Biomass(total) DAC2 H;

parms a0 a1 b0 b1 c0 c1

/*Biomass(stem)=a0+a1*(DAC2*H)+ε

Biomass(total)=d0+d1*(DAC2)+d2*(DAC2*H)+ε*/

/*σ2e(stem)=(DAC2*H)1.806,

σ2e (total)=(DA*C2*H)1.998*/

Biomass(stem)=a0+a1*(DAC2*H)+ε

bfy7=a1*(d2)+a2*(dh)+a3*(d2h);

resid.Biomass(stem)=resid.Biomass(stem)/(((DAC2*H)**1.806)**0.5);

/*weigths, I recommend Blaboa-Murias 2006, For.Eco.Mang. for details*/

e1 = actual. Biomass(stem) - pred. Biomass(stem);

/*so on for all components*/

Biomass(total)=a0+a1*(DAC2*H)+

b0+b1*(DAC2*H)+

c0+c1*(DAC2);

resid.Biomass(total)=resid.Biomass(total)/((DAC2H**1.998)**0.5);

e4 = actual.Biomass(total) - pred.Biomass(total);

outvars e1 ......... e4;

fit Biomass(stem)Biomass(branches) Biomass(foliaje) Biomass(total)

start=( a0 XX a1 XX b0 XX b1 XX c0 XX c1 XX)/ /* XX start values*/

sur outs=Smatrix outest=coeff cov out=values outpredict collin;

quit;

run;

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