Archive for Lebesgue integration

Quadrature methods for evidence approximation

Posted in Statistics with tags , , , , , on November 13, 2009 by xi'an

Two papers written by astronomers have been recently posted on arXiv about (new) ways to approximate evidence. Since they both perceive those approximations as some advanced form of quadrature, they are close enough that a comparison makes sense.

The paper by Rutger van Haasteren uses a Voronoi tessellation to represent the evidence as

\int f(x) dx \approx \sum f(x_i) O_i

when the x_i‘s are simulated from the normalised version of f and the O_i‘s are the associated Voronoi cells. This approximation converges (even when the x_i‘s are not simulated from the right distribution) but it cannot be used in practice because of the cost of the Voronoi tessellation. Instead, Rutger van Haasteren suggests using a sort of an approximate HPD region F_t and its volume, V_t, along with an harmonic mean within the HPD region:

\int f(x) dx \approx V_t N \big/ \sum_{x_i \in F_t} 1/f(x_i)

where N is the total number of simulations. So in the end this solution is actually the one proposed in our paper with Darren Wraith, as described in this earlier post! It is thus nice to see an application of this idea in a realistic situation, with performances that compare with nested sampling in its MultiNest version of Feroz, Hobson and Bridges. (This is especially valuable when considering that nested sampling is often presented as the only solution to approximating evidence.)

The second paper by Martin Weinberg also adopt a quadrature perspective, while integrating in the Lebesgue sense rather than in the Riemann sense. This perspective applies to nested sampling even though John Skilling does not justify nested sampling that way but  Martin Weinberg also shows that the (infamous) harmonic mean estimator also is a Lebesgue-quadrature approximation. The solution proposed in the paper is a different kind of truncation on the functional values, that relates more to nested sampling and on which I hope to report more thoroughly later.