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Berger 2000

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Berger 2000

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Berger 2000

  1. 1. BAYESIAN ANALYSIS: A LOOK AT TODAYAND THOUGHTS OF TOMORROW Francesca Ferraro 21 April 2015
  2. 2. Progress ! “ Boom” 1990-1999 ! Areas: engineering, medicine, archaeology, gentics and law. ! Areas of statistics: decision analysis and decision theory, biostatistics, generalized linear model.
  3. 3. Different approaches !  Objective Bayesian Analysis ! Subjective Bayesian Analysis ! Robust Bayesian Analysis ! Frequentist Bayes Analysis ! Quasi Bayesian Analysis
  4. 4. Objective Bayesian Analysis !  The first Bayesians, Bayes (1763) and Laplace (1812)), performed Bayesian analysis using a constant prior distribution for unknown parameters " “Inverse probability” !  The most familiar element of the objective Bayesian school is the use of objective prior distributions, designed to be minimally informative in some sense. The most famous of these is the Jeffreys-rule prior (Reference priors, Maximum entropy priors) !  The main advantage is that it is easy to use, but, on the other hand the objective Bayesian procedures often use improper prior distributions, and so do not automatically have desirable Bayesian properties, such as coherency.
  5. 5. Objective Bayesian Analysis Example •  Medical diagnosis (Mossman and Berger (2001)): Within a population for which p0=Pr(Disease D) , a diagnostic test results in either a Positive (+) or Negative (–) reading. Let p1 = Pr(+ | patient has D) and p2 = Pr(+ | patient does not have D). By Bayes’ theorem, •  In practice, the pi are typically unknown but, for i = 0, 1, 2, there are available (independent) data, xi, having Binomial(xi | ni, pi) densities. It is desired to find a 100(1 − α)% confidence set for θ.
  6. 6. Solution ! Use the standard Jeffreys-rule priors for pi ! Draw random pi from the posterior distributions, i = 0, 1, 2, that results from using the Jeffreys-rule priors with the data. ! Compute the associated !  Repeat this process 10,000 times. ! The α/2 and 1 − α/2 quantiles of these 10,000 generated θ form the desired confidence limits
  7. 7. Subjective Bayesian Analysis The “soul” of Bayesian statistics. Is it simple to use and coherent? ! To elicit all features of a subjective prior π(θ), one must infinitely accurately specify a infinite number of things. In practice, only a modest number of (never fully accurate) subjective elicitations are possible, so practical Bayesian analysis must somehow construct the entire prior distribution π(θ) from these elicitations. ! Example 2. One common method of turning elicitations into full priors is to use conjugate priors. But conjugate priors are model-dependent so, depending on the experiment designed to study θ, the subjective Bayesian following this “prior completion” strategy would be constructing different priors for the same θ, clearly incoherent
  8. 8. Robust Bayesian Analysis It works with classes of models and classes of prior distributions which reflecting the uncertainty remaining after the (finite) elicitation efforts ! Robust Bayesian analysis is a coherent system that is implementable in practice. In this approach subjective specifications are intervals and the analysis considers all priors compatible with these specifications and results in intervals as the posterior conclusions. ! Unfortunately, the interval specifications that are easy to work with seem to result in posterior intervals that are too wide for use in practice, and refining the class of prior distributions to eliminate unreasonable priors leads to computational challenges that limit applicability.
  9. 9. Frequentist Bayes Analysis Today, an increasing number of statisticians envisage that the unified foundation will be a mix of Bayesian and frequentist ideas. !  The language of statistics will be Bayesian ! From a methodological perspective, it is becoming clear that both Bayesian and frequentist methodology is going to be important ! There are an increasing number of examples in which frequentist arguments yield satisfactory answers quite directly, whereas Bayesian analysis requires a formidable amount of extra work.
  10. 10. Quasi Bayesian Analysis In this type of analysis, priors are chosen in various ad hoc fashions, including choosing vague proper priors, choosing priors to “span” the range of the likelihood, and choosing priors with tuning parameters that are adjusted until the answer “looks nice.” ! In normal hierarchical models with a “higher-level” variance V, it is quite common to use the vague proper prior density . ! This procedure is to use conjugate priors, with parameters chosen so that the prior is more spread out than the likelihood function but is roughly centered in the same region. ! The third quasi-Bayesian procedure is to write down proper (often conjugate) priors with unspecified parameters, and then treat these parameters as “tuning” parameters to be adjusted until the answer “looks nice.”
  11. 11. Computational Technique Most Bayesian computation is focused on calculation of posterior expectations, which are typically integrals of one to thousands of dimensions. Another common type of Bayesian computation is calculation of the posterior mode. ! La Place approximation ! Monte Carlo sampling ! MCMC

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