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Unformatted text preview: 7 The multivariate normal model Up until now all of our statistical models have been univariate models, that is, models for a single measurement on each member of a sample of individuals or each run of a repeated experiment. However, datasets are frequently multi- variate , having multiple measurements for each individual or experiment. This chapter covers what is perhaps the most useful model for multivariate data, the multivariate normal model, which allows us to jointly estimate population means, variances and correlations of a collection of variables. After first cal- culating posterior distributions under semiconjugate prior distributions, we show how the multivariate normal model can be used to impute data that are missing at random. 7.1 The multivariate normal density Example: Reading comprehension A sample of twenty-two children are given reading comprehension tests before and after receiving a particular instructional method. Each student i will then have two scores, Y i, 1 and Y i, 2 denoting the pre- and post-instructional scores respectively. We denote each students pair of scores as a 2 1 vector Y i , so that Y i = Y i, 1 Y i, 2 = score on first test score on second test . Things we might be interested in include the population mean , E[ Y ] = E[ Y i, 1 ] E[ Y i, 2 ] = 1 2 and the population covariance matrix , = Cov[ Y ] = E[ Y 2 1 ]- E[ Y 1 ] 2 E[ Y 1 Y 2 ]- E[ Y 1 ]E[ Y 2 ] E[ Y 1 Y 2 ]- E[ Y 1 ]E[ Y 2 ] E[ Y 2 2 ]- E[ Y 2 ] 2 = 2 1 1 , 2 1 , 2 2 2 , P.D. Hoff, A First Course in Bayesian Statistical Methods , Springer Texts in Statistics, DOI 10.1007/978-0-387-92407-6 7, c Springer Science+Business Media, LLC 2009 106 7 The multivariate normal model where the expectations above represent the unknown population averages. Having information about and may help us in assessing the effectiveness of the teaching method, possibly evaluated with 2- 1 , or the consistency of the reading comprehension test, which could be evaluated with the correlation coefficient 1 , 2 = 1 , 2 / p 2 1 2 2 . The multivariate normal density Notice that and are both functions of population moments , or population averages of powers of Y 1 and Y 2 . In particular, and are functions of first- and second-order moments: first-order moments: E[ Y 1 ] , E[ Y 2 ] second-order moments: E[ Y 2 1 ] , E[ Y 1 Y 2 ] , E[ Y 2 2 ] Recall from Chapter 5 that a univariate normal model describes a population in terms of its mean and variance ( , 2 ), or equivalently its first two moments (E[ Y ] = , E[ Y 2 ] = 2 + 2 ). The analogous model for describing first- and second-order moments of multivariate data is the multivariate normal model....
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