Archive for Wally Gilks

ARS: when to update?

Posted in Books, Kids, Statistics, University life with tags , , , , , on May 25, 2017 by xi'an

An email I got today from Heng Zhou wondered about the validity of the above form of the ARS algorithm. As printed in our book Monte Carlo Statistical Methods. The worry is that in the original version of the algorithm the envelope of the log-concave target f(.) is only updated for rejected values. My reply to the question is that there is no difference in the versions towards returning a value simulated from f, since changing the envelope between simulations does not modify the accept-reject nature of the algorithm. There is no issue of dependence between the simulations of this adaptive accept-reject method, all simulations remain independent. The question is rather one about efficiency, namely does it pay to update the envelope(s) when accepting a new value and I think it does because the costly part is the computation of f(x), rather than the call to the piecewise-exponential envelope. Correct me if I am wrong!

adaptive rejection sampling with fixed number of nodes

Posted in Books, Statistics, University life with tags , , , , , on October 8, 2015 by xi'an

This paper was arXived today by Martino and Louzada. It starts from the adaptive rejection sampling algorithm of Gilks and Wild (1993), which provided an almost automatic random generator for univariate log-concave target probability densities. By creating an envelope of the true target based on convexity. This algorithm was at the core of BUGS in its early days. And makes a good example of accept reject algorithm that I often use in class. It is however not so popular nowadays because of the unidimensional constraint. And because it is not particularly adequate for simulating a single realisation from a given target. As is the case when used in a Gibbs sampler. The authors only consider here the issue of the growing cost of running the adaptive rejection sampling algorithm, which is due to the update of the envelope at each rejection. They however fail to quantify this increase, which involves (a) finding the interval where the current rejection lies, among n existing intervals, which is of order O(n), and (b) creating both modified envelopes on both new intervals, which is of order O(1). The proposal of the authors, called cheap adaptive rejection sampling, settles for a fixed number τ of support points (and intervals), solely swapping a rejected point with the closest support point when this improves the acceptance rate. The test for swapping involves first computing the alternative target (on a pair of intervals) and the corresponding normalising constant, while swapping the rejected point with the closest support point involves finding the closest point, which is of order O(log τ). I am rather surprised that the authors do not even mention the execution time orders, resorting instead to a simulation where the gain brought by their proposal is not overwhelming. There is also an additional cost for figuring out the fixed number τ of support points, another issue not mentioned in the paper.