r - 如何在 NLME 中选择协变量的起始值

Question

library(nlme)
Loblolly$age2 <- as.factor(ifelse(Loblolly$age < 12.5, 0, 1))

在这里我定义了一个我感兴趣的二元协变量。

model <- nlme(height ~ (R0) + 1,
               data = Loblolly,
               fixed = list(R0 ~ 1 + (age2)),
               random = list(Seed = pdDiag(list(R0 ~ 1))),
               start = list(fixed = c(R0 = -8.5, age2 = 1)))

运行这个给我错误，

Error in nlme.formula(height ~ (R0) + 1, data = Loblolly, fixed = list(R0 ~  : 
  step halving factor reduced below minimum in PNLS step

更改起始值后，它工作正常。

model2 <- nlme(height ~ (R0) + 1,
               data = Loblolly,
               fixed = list(R0 ~ 1 + (age2)),
               random = list(Seed = pdDiag(list(R0 ~ 1))),
               start = list(fixed = c(R0 = 0, age2 = 30)), verbose=TRUE)

为 age2 选择起始值的方法有哪些？我考虑过使用 nls2 拟合非线性最小二乘模型，但这也需要指定一组起始值。

我在想也许我可以绘制数据 height ~ age2，但由于 age2 是二进制的...我不确定如何去做。

Answer 1

像这样尝试 lm:

fm.lm <- lm(height ~ age2, Loblollly) # modified Loblolly as per question
st <- coef(fm.lm)
names(st)[1] <- "R0"
nlme(height ~ (R0) + 1, data = Loblolly,
               fixed = list(R0 ~ 1 + (age2)),
               random = list(Seed = pdDiag(list(R0 ~ 1))),
               start = list(fixed = st))

给予:

Nonlinear mixed-effects model fit by maximum likelihood
  Model: height ~ (R0) + 1 
  Data: Loblolly 
  Log-likelihood: -305.1093
  Fixed: list(R0 ~ 1 + (age2)) 
R0.(Intercept)       R0.age21 
      12.96167       36.80548 

Random effects:
 Formula: R0 ~ 1 | Seed
        R0.(Intercept) Residual
StdDev:   0.0002791602 9.145988

Number of Observations: 84
Number of Groups: 14