R Under development (unstable) (2019-06-19 r76722) -- "Unsuffered Consequences"
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> library(bbmle)
Loading required package: stats4
> old_opt <- options(digits=3)
> tracelevel <- 0
> 
> ## source("~/lib/R/pkgs/bbmle/pkg/R/mle.R
> 
> set.seed(1002)
> X <- rexp(1000, rate = 0.0001)
> f <- function(X, rate) {
+   if (tracelevel>0 && rate<0) cat("rate<0: ",rate,"\n")
+   -sum(dexp(X, rate = rate, log = TRUE))
+ }
> if (FALSE) {
+   ## L-BFGS-B violates bounds, and gets stuck at lower bound
+   m <- mle2(minuslogl = f,
+             data = list(X = X),
+             start = list(rate = 0.01),
+             method = "L-BFGS-B",
+             control = list(trace = tracelevel, 
+                            parscale = 1e-4),
+             lower = c(rate = 1e-9))
+ 
+   profile(m, std.err=0.0001) ## finds new optimum
+ 
+   fsc <- function(X, rate) {
+     -sum(dexp(X, rate = rate*1e-4, log = TRUE))
+   }
+   msc <- mle2(minuslogl = fsc,
+             data = list(X = X),
+             start = list(rate = 100),
+             method = "L-BFGS-B",
+             control = list(trace = tracelevel),
+             lower = c(rate = 1e-5))
+ 
+   ## does it work if we scale by hand?
+   ##   no, identical problem
+ }
> 
> ## works fine with a better starting point
> m <- mle2(minuslogl = f,
+           data = list(X = X),
+           start = list(rate = 0.001),
+           method = "L-BFGS-B",
+           control = list(trace = tracelevel,
+                          parscale=1e-4),
+               lower = c(rate = 1e-9))
> vcov(m)
         rate
rate 1.05e-11
> confint(m)
   2.5 %   97.5 % 
9.61e-05 1.09e-04 
> 
> 
> ## works OK despite warnings about 1-dimensional opt. with N-M
> (m0 <- mle2(minuslogl = f,
+           data = list(X = X),
+           start = list(rate = 0.01),
+           method = "Nelder-Mead",
+           control = list(trace = tracelevel, parscale = 1e-4)))

Call:
mle2(minuslogl = f, start = list(rate = 0.01), method = "Nelder-Mead", 
    data = list(X = X), control = list(trace = tracelevel, parscale = 1e-04))

Coefficients:
    rate 
0.000102 

Log-likelihood: -10188 
Warning message:
In optim(par = c(rate = 0.01), fn = function (p)  :
  one-dimensional optimization by Nelder-Mead is unreliable:
use "Brent" or optimize() directly
> vcov(m0)
         rate
rate 1.05e-11
> 
> confint(m0)
   2.5 %   97.5 % 
0.000096 0.000109 
> confint(m0,method="quad")
   2.5 %   97.5 % 
0.000096 0.000109 
> ## very similar (good quadratic surface, not surprising)
> 
> m1 <- mle2(minuslogl = f,
+           data = list(X = X),
+           start = list(rate = 0.01),
+           method = "BFGS",
+           control = list(trace = tracelevel, parscale = 1e-4))
There were 11 warnings (use warnings() to see them)
> 
> 
> ## gets stuck? will have to investigate ...
> m2 <- mle2(minuslogl = f,
+            data = list(X = X),
+            start = list(rate = 0.01),
+            optimizer = "optimize",
+            lower=1e-9,upper=0.1)
> 
> vcov(m2)
         rate
rate 1.41e-11
> options(old_opt)
> 
> proc.time()
   user  system elapsed 
  1.528   0.100   1.762