Archive for likelihood ratio

Statistical Inference

Posted in Books, Statistics, University life with tags , , , , , , , , , on November 16, 2010 by xi'an

Following the publication of several papers on the topic of integrated evidence (about competing models), Murray Aitkin has now published a book entitled Statistical Inference and I have now finished reading it. While I appreciate the effort made by Murray Aitkin to place his theory within a coherent Bayesian framework, I remain unconvinced of the said coherence, for reasons exposed below.

The main chapters of the book are Chapter 2 about the “Integrated Bayes/likelihood approach” and Chapter 4 about the “Unified analysis of finite populations”, Chapter 7 also containing a new proposal about “Goodness of fit and model diagnostics”. Chapter 1 is a nice introduction to frequentist, likelihood and Bayesian approaches to inference and the four remaining chapters are applications of Murray Aitkin‘s principles to various models.  The style of the book is quite pleasant although slightly discursive in what I (a Frenchman!) would qualify as an English style in that it is often relying on intuition to develop concepts. I also think that the argument of being close to the frequentist decision (aka the p-value) too often serves as a justification in the book (see, e.g., page 43 “the p-value has a direct interpretation as a posterior probability”). As an aside, Murray Aitkin is a strong believer in plotting cdfs rather than densities to provide information about a distribution and hence cdf plots abound throughout the book.  (I counted 82 pictures of them.) While the book contains a helpful array of examples and datasets, the captions of the (many) figures are too terse for my taste: The figures are certainly not self-contained and even with the help of the main text they do not always make complete sense. Continue reading →

Posterior likelihood

Posted in pictures, R, Statistics, Travel with tags , , , , on March 6, 2010 by xi'an

At the Edinburgh mixture estimation workshop, Murray Aitkin presented his proposal to compare models via the posterior distribution of the likelihood ratio.

\dfrac{L_1(\theta_1|x)}{L_2(\theta_2|x)}

As already commented in a post last July, the positive aspect of looking at this quantity rather than at the Bayes factor is that the priors are then allowed to be improper if one simulates from the posteriors for each model, as in Aitkin et al. (2007). My overall feeling has not changed though, namely the ratio should be instead considered under the joint posterior of (\theta_1,\theta_2), which is [proportional to]

p_1 m_1(x) \pi_1(\theta_1|x) \pi_2(\theta_2)+p_2 m_2(x) \pi_2(\theta_2|x) \pi_1(\theta_1)

instead of the product of both posteriors. This of course makes a whole difference, as shown on the next R graph that compares the distribution of the likelihood ratio under the true posterior and under the product of posteriors (when comparing a Poisson model against a negative binomial with m=5 successes trials, when x=3). The joint simulation produces a much more supportive argument in favour of the negative binomial model, when compared with the product of the posteriors.

Obviously, this joint perspective also cancels the appeal of the approach under improper priors.