R help - Apr 2007 - Likelihood returning inf values to optim(L-BFGS-B) other

Hello,
A couple of ideas...


Im not clear on your whole problem however...

Consider  making use of the lgamma function, which returns the natural 
log of the gamma function. This may help.
The gamma function gets awfully, big very fast.

Also multivariable likelihoods can be bumpy like a mountain range, with 
minor peaks and valleys. It is possible that your likelihood has such a 
shape. Maybe each iteration Xn is trying to get closer to the main 
peak, but instead goes up the ridge of a valley and gets lost, 
ultimately reaching a boundary of the region.

You could try starting at a variety of locations. Possibly many 
hundreds of starting points, randomly selected from within your region.
Then examine the ending point for each starting point. 

If you do have a bumpy likelihood surface you might have to start very 
close to the actual maximum to get there. Surface plots might help, 
setting some variables to a constant. I know in 4D this will be tough.

Here is a useful reference that helped me recently with a similar 
maximization problem:

"Computational Statistics"
by Geof H. Givens and Jennifer A. Hoeting

They have R-code examples here:
http://www.stat.colostate.edu/computationalstatistics/



Good luck!

Joe Liddle
University of Alaska Southeast