Question

I am trying to use R to estimate a multinomial logit model with a manual specification. I have found a few packages that allow you to estimate MNL models here or here.

I’ve found some other writings on “rolling” your own MLE function here. However, from my digging around – all of these functions and packages rely on the internal optim function.

In my benchmark tests, optim is the bottleneck. Using a simulated dataset with ~16000 observations and 7 parameters, R takes around 90 seconds on my machine. The equivalent model in Biogeme takes ~10 seconds. A colleague who writes his own code in Ox reports around 4 seconds for this same model.

Does anyone have experience with writing their own MLE function or can point me in the direction of something that is optimized beyond the default optim function (no pun intended)?

If anyone wants the R code to recreate the model, let me know – I’ll glady provide it. I haven’t provided it since it isn’t directly relevant to the problem of optimizing the optim function and to preserve space…

EDIT: Thanks to everyone for your thoughts. Based on a myriad of comments below, we were able to get R in the same ballpark as Biogeme for more complicated models, and R was actually faster for several smaller / simpler models that we ran. I think the long term solution to this problem is going to involve writing a separate maximization function that relies on a fortran or C library, but am certainly open to other approaches.

Answer 1

Tried with the nlm() function already? Don’t know if it’s much faster, but it does improve speed. Also check the options. optim uses a slow algorithm as the default. You can gain a > 5-fold speedup by using the Quasi-Newton algorithm (method=”BFGS”) instead of the default. If you’re not concerned too much about the last digits, you can also set the tolerance levels higher of nlm() to gain extra speed.

f <- function(x) sum((x-1:length(x))^2)

a <- 1:5

system.time(replicate(500,
     optim(a,f)
))
   user  system elapsed 
   0.78    0.00    0.79 

system.time(replicate(500,
     optim(a,f,method="BFGS")
))
   user  system elapsed 
   0.11    0.00    0.11 

system.time(replicate(500,
     nlm(f,a)
))
   user  system elapsed 
   0.10    0.00    0.09 

system.time(replicate(500,
      nlm(f,a,steptol=1e-4,gradtol=1e-4)
))
   user  system elapsed 
   0.03    0.00    0.03