Re: Generalised bisection

[Ealdwulf Wuffinga <ealdwulf@googlemail.com>]

On Thu, Mar 12, 2009 at 6:02 PM, Steven Tweed <orthochronous@gmail.com> wrote:

> I haven't even looked at the source code so a description of the
> mathematical algorithm would help, but I'll just point out that
> underflow (in the case of working with probabilities) and overflow
> (when working with their negated logarithms) is inherent in most
> multi-step Bayesian algorithms. The only solution is to rescale things
> as you go so that things stay in a "computable" range. (You're almost
> never interested in absolute probabilities anyway but rather relative
> probabilities or, in extreme cases, just the biggest probability, so
> rescaling isn't losing any useful information.)

Are you sure you aren't thinking of when you are using fixed point? I
was under the impression
that Bayesian algorithms usually worked okay in floating point.

One issue in BBChop which should be easy to fix, is that I use a dumb
way of calculating Beta functions. These
are ratios of factorials, so the subexpressions get stupidly big very
quickly. But I don't think that is the only problem.


Ealdwulf