[ADMB Users] very small number math

[Fowler, Mark]

Yes, and I use epsilon for proper parameters. The situation I'm
addressing is 'non-ADMB' iterative mortality (F) estimates. These are
just loop functions to estimate F's to create population and mortality
matrices. Obviously candidates as parameters, but I needed to keep them
as originally written. I was translating a particular type of VPA
written in a language called ACON to R and ADMB. Only the reference year
abundances are estimated with a serious optimizing function in the
original model (to reasonably estimate catchabilities).  Initially this
served for truthing until I finished the translations. Hence the issue
of dealing with extremely small values are the profusion of F loops. And
I'm doing these in an equivalent manner to your suggestion, I just don't
use epsilon per se because I like to restrict it to parameters.

I've since been keeping the loops while comparing vulnerabilities to
local minima across the three programs, so only the reference year
abundances are optimized directly. Both optim and the ACON optimizer
(NLLS) clearly work left to right on parameter sets. The optim function
gets trapped in local minima with high uninformative constants for
priors on a vector that grades high to low. Starting left-most they go
up, which is correct relative to numbers to the right but wrong in
absolute terms. NLLS starts out similarly (run with few iterations you
get the same answer) but doesn't stay there, so maybe breaks the
left-to-right rule at some point to explore the range, or has some
threshold point at which it gives up on the first parameter. So far I'm
stymied with ADMB. I'm thinking maybe the F estimations confound
tracking of gradients? 


>	Mark Fowler
		Population Ecology Division
>	Bedford Inst of Oceanography
>	Dept Fisheries & Oceans
>	Dartmouth NS Canada
		B2Y 4A2
		Tel. (902) 426-3529
		Fax (902) 426-9710
		Email Mark.Fowler at dfo-mpo.gc.ca
		Home Tel. (902) 461-0708
		Home Email mark.fowler at ns.sympatico.ca


-----Original Message-----
From: users-bounces at admb-project.org
[mailto:users-bounces at admb-project.org] On Behalf Of dave fournier
Sent: March 8, 2013 11:52 AM
To: users at admb-project.org
Subject: Re: [ADMB Users] very small number math

This is proabably a good use of phases. In the early phase you add
something

like

                        x/(eps+y)

and let eps get smaller or 0 in a later phase.
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