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2013 | OriginalPaper | Buchkapitel

3. Probabilities

verfasst von : Uffe B. Kjærulff, Anders L. Madsen

Erschienen in: Bayesian Networks and Influence Diagrams: A Guide to Construction and Analysis

Verlag: Springer New York

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Abstract

The fact that the structure of a probabilistic network can be characterized as a DAG derives from basic axioms of probability calculus leading to recursive factorization of a joint probability distribution into a product of lower-dimensional conditional probability distributions. First, any joint probability distribution can be decomposed (or factorized) into a product of conditional distributions of different dimensionality, where the dimensionality of the largest distribution is identical to the dimensionality of the joint distribution. This gives rise to a densely connected DAG. Second, statements of local conditional independences manifest themselves as reductions of dimensionalities of some of the conditional probability distributions. Most often, these independence statements give rise to dramatic reductions of complexity of the DAG such that the resulting DAG appears to be quite sparse.

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Fußnoten
1
The dimensionality of a function is defined as the number of variables over which the function is defined.
 
2
See Sect. 3.3.3 on page 50 for more on marginalization.
 
3
Note that the two interpretations are consistent. See Sect. 3.3.3 on page 50 for details on marginalization.
 
4
See Sect. 3.3.3 for a definition of combination of potentials.
 
5
As defined in Sect. 2.​2 on page 20.
 
6
The misuse is concerned with differences in dimensionalities of \(P(X\vert Y,Z)\) and \(P(X\vert Z)\).
 
7
See (8.1) for the number of possible DAGs on n vertices.
 
8
For notational convenience, we assume (without loss of generality) that v 1, , v n is a topological ordering.
 
9
This result is probably easiest to acknowledge from the directed Markov criterion: The graph \({({\mathcal{G}}_{\mathrm{An}(\{v\}\cup \mathrm{nd}(v)\cup \mathrm{pa}(v))})}^{m}\) obviously excludes all descendants of v forcing all paths to v to go through pa(v).
 
10
Special cases in which variables are independent only for particular values of some other variable(s) might exist. Such context-specific independence properties obviously cannot be captured by a DAG. Thus, the one-to-one correspondence should be understood with respect to independence statements on the “level of variables.”
 
Metadaten
Titel
Probabilities
verfasst von
Uffe B. Kjærulff
Anders L. Madsen
Copyright-Jahr
2013
Verlag
Springer New York
DOI
https://doi.org/10.1007/978-1-4614-5104-4_3