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Bayes Factor

Bayes Factor. Based on Han and Carlin (2001, JASA). Model Selection. Data: y : finite set of competing models : a distinct unknown parameter vector of dimension n j corresponding to the j th model Prior : all possible values for

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Bayes Factor

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  1. Bayes Factor Based on Han and Carlin (2001, JASA)

  2. Model Selection • Data: y • : finite set of competing models • : a distinct unknown parameter vector of dimension nj corresponding to the jth model • Prior • : all possible values for • : collection of all model specific

  3. Model Selection • Posterior probability • A single “best” model • Model averaging • Bayes factor: Choice between two models

  4. Estimating Marginal Likelihood • Marginal likelihood • Estimation • Ordinary Monte Carlo sampling • Difficult to implement for high-dimensional models • MCMC does not provide estimate of marginal likelihood directly • Include model indicator as a parameter in sampling • Product space search by Gibbs sampling • Metropolis-Hastings • Reversible Jump MCMC

  5. Product space search • Carlin and Chib (1995, JRSSB) • Data likelihood of Model j • Prior of model j • Assumption: • M is merely an indicator of which is relevant to y • Y is independent of given the model indicator M • Proper priors are required • Prior independence among given M

  6. Product space search • The sampler operates over the product space • Marginal likelihood • Remark:

  7. Search by Gibbs sampler

  8. Bayes Factor • Provided the sampling chain for the model indicator mixes well, the posterior probability of model j can be estimated by • Bayes factor is estimated by

  9. Choice of prior probability • In general, can be chosen arbitrarily • Its effect is divided out in the estimate of Bayes factor • Often, they are chosen so that the algorithm visits each model in roughly equal proportion • Allows more accurate estimate of Bayes factor • Preliminary runs are needed to select computationally efficient values

  10. More remarks • Performance of this method is optimized when the pseudo-priors match the corresponding model specific priors as nearly as possible • Draw back of the method • Draw must be made from each pseudo prior at each iteration to produce acceptably accurate results • If a large number of models are considered, the method becomes impractical

  11. Metropolized product space search • Dellaportas P., Forster J.J., Ntzoufras I. (2002). On Bayesian Model and Variable Selection Using MCMC.  Statistics and Computing, 12, 27-36. • A hybrid Gibbs-Metropolis strategy • Model selection step is based on a proposal moving between models

  12. Metropolized Carlin and Chib (MCC)

  13. Advantage • Only needs to sample from the pseudo prior for the proposed model

  14. Reversible jump MCMC • Green (1995, Biometrika) • This method operates on the union space • It generates a Markov chain that can jump between models with parameter spaces of different dimensions

  15. RJMCMC

  16. Using Partial Analytic Structure (PAS) • Godsill (2001, JCGS) • Similar setup as in the CC method, but allows parameters to be shared between different models. • Avoids dimension matching

  17. PAS

  18. Marginal likelihood estimation (Chib 1995)

  19. Marginal likelihood estimation (Chib 1995) • Let When all full conditional distributions for the parameters are in closed form

  20. Chib (1995) • The first three terms on the right side are available in close form • The last term on the right side can be estimated from Gibbs steps

  21. Chib and Jeliazkov (2001) • Estimation of the last term requires knowing the normalizing constant • Not applicable to Metropolis-Hastings • Let the acceptance probability be

  22. Chib and Jeliazkov (2001)

  23. Example: linear regression model

  24. Two models

  25. Priors

  26. For MCC • h(1,1)=h(1,2)=h(2,1)=h(2,2)=0.5

  27. For RJMCMC • Dimension matching is automatically satisfied • Due to similarities between two models

  28. Chib’s method • Two block gibbs sampler • (regression coefficients, variance)

  29. Results • By numerical integration, the true Bayes factor should be 4862 in favor of Model 2

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