I have been using gnls with the weights argument (and varConstPower) to specify a variance structure for curve fits. In attempting to extract the parameters for the variance model I am seeing results I don't understand. When I simply display the model (or use "summary" on the model), I get what seem like reasonable values for both "power" and "const". When I actually try to extract the values, I get the same number for the "power", but a different (and less sensible) value for "const".
The simplest example I can come up with that shows the problem is as follows: #Set up data x = c(0,0,0,1,1,1,2,2,2,3,3,3,4,4,4,5,5,5,6,6,6,7,7,7); y0 = c(.1,.1,.1, .5,.5,.5, 1,1,1, 2,2,2, 4,4,4, 7,7,7, 9,9,9, 10,10,10); yp = c(0,.03,.05, 0,.05,.01, 0,.07,.03, .1,0,.2, .2,.1,0, .3,0,.1, 0,.3,.4, .3,.5,0); y = y0 + 4*yp; data = data.frame(x=x,y=y); #Run model library(nlme) model3 = try(gnls(y ~ SSfpl(x,A,B,xmid,scal),data=data,weights=varConstPower(const=1,power=0,form= ~fitted(.)))); #Examine results model3; #const = .6551298, power = .8913665 summary(model3); #const = .6551298, power = .8913665 coef(model3$modelStruct$varStruct)["power"]; # power = .8913665 coef(model3$modelStruct$varStruct)["const"]; #const = -0.4229219 coef.varFunc(model3$modelStruct$varStruct); #R can't seem to find this function, Splus can Any advice on what I am doing wrong would be appreciated. Hugh Rand Senior Scientist Amgen [EMAIL PROTECTED] ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html