Markov Chain Monte Carlo

In statistics, Markov chain Monte Carlo (MCMC) methods are a class of algorithms for sampling from a probability distribution based on constructing a Markov chain that has the desired distribution as its equilibrium distribution. The state of the chain after a number of steps is then used as a sample of the desired distribution. The quality of the sample improves as a function of the number of steps.

Random walk Monte Carlo methods make up a large subclass of MCMC methods.

When an MCMC method is used for approximating a multi-dimensional integral, an ensemble of "walkers" move around randomly. At each point where a walker steps, the integrand value at that point is counted towards the integral. The walker then may make a number of tentative steps around the area, looking for a place with a reasonably high contribution to the integral to move into next.

Random walk Monte Carlo methods are a kind of random simulation or Monte Carlo method. However, whereas the random samples of the integrand used in a conventional Monte Carlo integration are statistically independent, those used in MCMC methods are correlated. A Markov chain is constructed in such a way as to have the integrand as its equilibrium distribution.

Examples of random walk Monte Carlo methods include the following:

Unlike most of the current MCMC methods that ignore the previous trials, using a new algorithm the MCMC algorithm is able to use the previous steps and generate the next candidate. This training-based algorithm is able to speed-up the MCMC algorithm by an order of magnitude.

Interacting MCMC methodologies are a class of mean field particle methods for obtaining random samples from a sequence of probability distributions with an increasing level of sampling complexity. These probabilistic models include path space state models with increasing time horizon, posterior distributions w.r.t. sequence of partial observations, increasing constraint level sets for conditional distributions, decreasing temperature schedules associated with some Boltzmann-Gibbs distributions, and many others. In principle, any MCMC sampler can be turned into an interacting MCMC sampler. These interacting MCMC samplers can be interpreted as a way to run in parallel a sequence of MCMC samplers. For instance, interacting simulated annealing algorithms are based on independent Metropolis-Hastings moves interacting sequentially with a selection-resampling type mechanism. In contrast to traditional MCMC methods, the precision parameter of this class of interacting MCMC samplers is only related to the number of interacting MCMC samplers. These advanced particle methodologies belong to the class of Feynman-Kac particle models, also called Sequential Monte Carlo or particle filter methods in Bayesian inference and signal processing communities. Interacting MCMC methods can also be interpreted as a mutation-selection genetic particle algorithm with MCMC mutations.

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