Statistical models for spatially-dependent, discrete-valued
observations associated with areal units (i.e., discrete space) are
used routinely in areas of application ranging from disease mapping to
small area estimation to image analysis. Despite a rich literature on
both classical and Bayesian models for this setting, theoretical and
computational considerations remain, particularly in the Bayesian
setting when complete posterior inference is desired. To address the
computational and theoretical concerns of existing models, we propose
a novel modeling framework based on a mixture of directed graphical
models (MDGMs). The components of the mixture, directed graphical
models, can be represented by directed acyclic graphs (DAGs) and are
computationally quick to evaluate. The DAGs representing the mixture
components are selected to correspond to an undirected graphical
representation of an assumed spatial contiguity/dependence structure
for the areal units which underlies the specification of traditional
modeling approaches for discrete spatial processes such as Markov
random fields (MRFs). We introduce the concepts of weak and strong
compatibility to show how an undirected graph can be used as a
template for the structural dependencies between areal units to create
sets of DAGs which, as a collection, preserve the structural
dependencies and conditional independences represented in the template
undirected graph. We then introduce three classes of compatible DAGs
and corresponding algorithms for fitting MDGMs based on these classes.
In addition, we compare MDGMs to MRFs and a popular Bayesian MRF model
approximation used in high-dimensional settings in a series of
simulations and an analysis of ecometrics data collected as part of
the Adolescent Health and Development in Context Study. This
presentation is based on joint work with Brandon Carter.
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