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From S├ębastien Brisard <>
Subject [math] MATH-692 "sub"-issue?
Date Mon, 31 Oct 2011 07:21:49 GMT
Hello all,
as agreed in MATH-692, I've looked into making
more robust.
One of the main issues is the fact that the present implementation
does not return the expected value when the cumulative distribution
has a plateau.
This should be recorded as a bug, but there is nothing wrong with the
logic of the current implementation of inverseCumulativeDistribution.
Rather, the bracketing method and the solver can be slightly altered
to return the expected value. As an example, I did quick and dirty
modifications, which seem to solve the problem:
1. The contract for bracketing is to return [a, b] such as f(a) * f(b)
<= 0. If we require f(a) * f(b) < 0 (strict equality), we ensure that
the entire plateau is included.
2. In inverseCumulativeDistribution, I replaced the default solver
(Brent) by Bisection (because it's easier to alter if needed).
inverseCumulativeDistribution then returns the expected value (ie, the
lower-end of the plateau).

The above modifications are very dirty, and their description are very
vague for the moment. I will post diff files, as I think there is
space for discussion on these points. I would like to open a JIRA
ticket on this specific issue, in order not to clutter MATH-692 (which
is more design-oriented) too much. Especially, point 1 might be an
improvement on the current implementation of the bracketing method,
because the "plateau issue" is likely to occur elsewhere in CM.

Looking forward to reading your thoughts,

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