R help es - Mar 2017 - step halving factor reduced below minimum in PNLS step

Tree dbh haut Btot
1 35.00 18.90 0.357
2 25.00 16.60 0.214
3 23.00 19.50 0.173
4 13.50 15.60 0.060
5 20.00 18.80 0.134
6 23.00 17.40 0.137
7 29.00 19.90 0.428
8 17.60 18.20 0.100
9 31.00 25.30 0.514
10 26.00 23.50 0.273
11 13.00 13.00 0.031
12 32.00 20.70 0.356
13 28.00 28.50 0.349
14 15.00 18.20 0.068
15 19.00 14.90 0.110
16 33.00 14.90 0.260
17 24.00 19.10 0.160
18 22.00 19.20 0.204
19 39.00 25.20 0.724
20 30.00 26.60 0.386
Hello dear all:

I am using above data to run the following code;

library(nlme)

start <- coef(lm(Btot~I(dbh**2*haut),data=dat))

names(start) <- c("a","b")

model1 <-(nlme (Btot~a+b*dbh**2*haut, data=cbind(dat, g="a"),
fixed=a+b~1,
start=start,

groups=~g, weights=varPower(form=~dbh)))


I get regression parameters with the intercept being non-significant.
Therefore, I run the following code to obtain an equation without
intercept..


summary(nlme(Btot~b*dbh**2*haut, data=cbind(dat,g="a"), fixed=b~1,
start=start["b"], groups=~g, weights=varPower(form=~dbh)))


When I do run the last code, I get the following error:


Error in nlme.formula(Btot ~ b * dbh**2 * haut, data = cbind(dat, g =
"a"),
 :

  step halving factor reduced below minimum in PNLS step

Can someone help??


Best regards,

Santiago

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