**I**n the continuation of my earlier post on computing evidence, I read a very interesting paper by Merlise Clyde, Joyee Ghosh and Michael Littman, to appear in *JCGS*. It is called *Bayesian adaptive sampling for variable selection and model averaging*. The sound idea at the basis of the paper is that, when one is doing variable selection (i.e. exploring a finite if large state space) in setups where the individual probabilities of the models are known (up to a constant), it is not necessary to return to models that have been already visited. Hence the move to sample models without replacement called BAS (for Bayesian adaptive sampling) in the paper. The first part discusses the way to sample without replacement a state space whose elements and probabilities are defined by a binary tree, i.e.

(The connection with variable selection is that each level of the tree corresponds to the binary choice between including and excluding one of the variables. The tree thus has *2*^{k} endpoints/leaves for *k* potential variables in the model.) The cost in updating the probabilities is actually in *O(k)* if *k* is the number of levels, instead of *2*^{k} because most of the branches of the tree are unaffected by setting one final branch to probability zero. The second part deals with the adaptive and approximative issues.

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