Sensitivity Analysis

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Description

This function performs an elementary sensitivity analysis for two models regarding marginal posterior distributions and posterior inferences.

Usage

1
SensitivityAnalysis(Fit1, Fit2, Pred1, Pred2)

Arguments

Fit1

This argument accepts an object of class demonoid, iterquad, laplace, pmc, or vb.

Fit2

This argument accepts an object of class demonoid, iterquad, laplace, pmc, or vb.

Pred1

This argument accepts an object of class demonoid.ppc, iterquad.ppc, laplace.ppc, pmc.ppc, or vb.ppc.

Pred2

This argument accepts an object of class demonoid.ppc, iterquad.ppc, laplace.ppc, pmc.ppc, or vb.ppc.

Details

Sensitivity analysis is concerned with the influence from changes to the inputs of a model on the output. Comparing differences resulting from different prior distributions is the most common application of sensitivity analysis, though results from different likelihoods may be compared as well. The outputs of interest are the marginal posterior distributions and posterior inferences.

There are many more methods of conducting a sensitivity analysis than exist in the SensitivityAnalysis function. For more information, see Oakley and O'Hagan (2004). The SIR function is useful for approximating changes in the posterior due to small changes in prior distributions.

The SensitivityAnalysis function compares marginal posterior distributions and posterior predictive distributions. Specifically, it calculates the probability that each distribution in Fit1 and Pred1 is greater than the associated distribution in Fit2 and Pred2, and returns a variance ratio of each pair of distributions. If the probability is 0.5 that a distribution is greater than another, or if the variance ratio is 1, then no difference is found due to the inputs.

Additional comparisons and methods are currently outside the scope of the SensitivityAnalysis function. The BayesFactor function may also be considered, as well as comparing posterior predictive checks resulting from summary.demonoid.ppc, summary.iterquad.ppc, summary.laplace.ppc, summary.pmc.ppc, or summary.vb.ppc.

Regarding marginal posterior distributions, the SensitivityAnalysis function compares only distributions with identical parameter names. For example, suppose a statistician conducts a sensitivity analysis to study differences resulting from two prior distributions: a normal distribution and a Student t distribution. These distributions have two and three parameters, respectively. The statistician has named the parameters beta and sigma for the normal distribution, while for the Student t distribution, the parameters are named beta, sigma, and nu. In this case, the SensitivityAnalysis function compares the marginal posterior distributions for beta and sigma, though nu is ignored because it is not in both models. If the statistician does not want certain parameters compared, then differing parameter names should be assigned.

Robust Bayesian analysis is a very similar topic, and often called simply Bayesian sensitivity analysis. In robust Bayesian analysis, the robustness of answers from a Bayesian analysis to uncertainty about the precise details of the analysis is studied. An answer is considered robust if it does not depend sensitively on the assumptions and inputs on which it is based. Robust Bayes methods acknowledge that it is sometimes very difficult to come up with precise distributions to be used as priors. Likewise the appropriate likelihood function that should be used for a particular problem may also be in doubt. In a robust Bayesian analysis, a standard Bayesian analysis is applied to all possible combinations of prior distributions and likelihood functions selected from classes of priors and likelihoods considered empirically plausible by the statistician.

Value

This function returns a list with the following components:

Posterior

This is a J x 2 matrix of J marginal posterior distributions. Column names are "p(Fit1 > Fit2)" and "var(Fit1) / var(Fit2)".

Post.Pred.Dist

This is a N x 2 matrix of N posterior predictive distributions. Column names are "p(Pred1 > Pred2)" and "var(Pred1) / var(Pred2)".

Author(s)

Statisticat, LLC software@bayesian-inference.com

References

Berger, J.O. (1984). "The Robust Bayesian Viewpoint (with discussion)". In J. B. Kadane, editor, Robustness of Bayesian Analyses, p. 63–144. North-Holland, Amsterdam.

Berger, J.O. (1985). "Statistical Decision Theory and Bayesian Analysis". Springer-Verlag, New York.

Berger, J.O. (1994). "An Overview of Robust Bayesian Analysis (with discussion)". Test, 3, p. 5–124.

Oakley, J. and O'Hagan, A. (2004). "Probabilistic Sensitivity Analysis of Complex Models: a Bayesian Approach". Journal of the Royal Statistical Society, Series B, 66, p. 751–769.

Weiss, R. (1995). "An Approach to Bayesian Sensitivity Analysis". Journal of the Royal Statistical Society, Series B, 58, p. 739–750.

See Also

BayesFactor, IterativeQuadrature, LaplaceApproximation, LaplacesDemon, PMC, predict.demonoid, predict.iterquad, predict.laplace, predict.pmc, SIR, summary.demonoid.ppc, summary.iterquad.ppc, summary.laplace.ppc, summary.pmc.ppc, and VariationalBayes.

Examples

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#sa <- SensitivityAnalysis(Fit1, Fit2, Pred1, Pred2)
#sa

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