We will present general definitions and discussion in the context of finite-state chains on a state space ; translating to continuous state space such as involves slightly different notation without any change of substance.
Write for a proposal transition matrix on . The simplest case is where is symmetric (). In this case, given on we define a step of the associated Metropolis chain in words by
pick from and propose a move to ;
accept the move (i.e. set ) with probability , otherwise stay ().
This recipe defines the transition matrix of the Metropolis chain to be
Assuming is irreducible and strictly positive, then clearly is irreducible. Then since , symmetry of implies satisfies the detailed balance equations and so is reversible with stationary distribution .
The general case is where is an arbitrary transition matrix, and the acceptance rule becomes
accept a proposed move with probability .
The transition matrix of the Metropolis chain becomes
(11.5) |
To ensure irreducibility, we now need to assume connectivity of the graph on whose edges are the such that . Again detailed balance holds, because
The general case is often called Metropolis-Hastings – see Notes for terminological comments.
The abstract setup described below comes from Diaconis [124]. Think of each as a line, i.e. the set of points in a line.
Suppose we have a collection of subsets of state space , with . Write . Suppose for each we are given a probability distribution on , and suppose
(11.6) |
Write . Define a step of the line-sampling chain in words by
choose from ;
then choose from .
So the chain has transition matrix
We can rewrite this as
and then (11.6) makes it clear that . For irreducibility, we need the condition
the union over of the edges in the complete graphs |
(11.7) |
Note in particular we want the to be overlapping, rather than a partition.
This setting includes many examples of random walks on combinatorial sets. For instance, card shuffling by random transpositions (yyy cross-ref) is essentially the case where the collection of subsets consists of all -card subsets. In the setting, with target density , the Gibbs sampler is the case where the collection consists of all lines parallel to some axis. Taking instead all lines in all directions gives the hit-and-run sampler, for which a step from is defined as follows.
Pick a direction uniformly at random, i.e. a point on the surface on the unit ball.
Step from to , where is chosen with density proportional to
The term here arises as a Jacobean; see Liu [237] Chapter 8 for explanation and more examples in .