## adaptive incremental mixture MCMC

**S**adly, I missed this adaptive incremental mixture MCMC paper by my friends Florian Maire, Nial Friel, Antonietta Mira, and Adrian E. Raftery when it came out in JCGS in 2019. The core of the paper is about building a time-inhomogeneous mixture independent proposal, starting from an initial distribution and adding one component when hitting a point for which the ratio target / proposal is large, as this points out a part of the space that is not well-enough explored, while the other components do not change, except for a proportional decrease in the weights. This proposal reminded me of the inspiring paper of Gåsemyr (2003), which in some ways inspired our population Monte Carlo sampler. Obviously, there is a what-you-get-is-what-you-see drawback to the approach in that regions where this ratio is high may never be explored by the proposal, despite its adaptivity.

The added component is Normal, centred at the associated (accepted) proposed value ø and with covariance matrix a local estimate based on past iterations of the algorithm. And with weight proportional to the (powered) target density at ø, which does not require a normalising constant. The method however requires setting a certain number of calibration parameters like the power γ for the weight, the lower bound M for the ratio target to proposal, the rate of diminishing adaptation (which is also needed for ergodicity à la Roberts and Rosenthal (2007)). And the implicit choice of a particular parameterisation for the Normal mixture to be close enough to the target. In the posted experiments, the number of components in the mixture does not grow to unmanageable figures, but a further adaption could be in removing components that are inactive or leading to systematic rejection as we did in the population Monte Carlo paper.

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