## Archive for Cornell University

## Bill’s 80th birthday

Posted in Statistics, Travel, University life with tags 80th birthday, Cornell University, flight, frequentist inference, James-Stein estimator, mathematical statistics, New York, Pitman nearness, Rutgers University, shrinkage estimation, William Strawderman on March 30, 2022 by xi'an## David Cox (1924-2022)

Posted in Books, Statistics, University life with tags ABC, Applied probabillity, Applied stochastic processes, Biometrika, Birmingham, Copley Medal, Cornell University, Cox process, CREST, David Cox, England, experimental design, FRS, Glasgow, Guy Medal in Gold, International Prize in Statistics, Ithaca, Kettering Prize for Cancer Research, mathematical statistics, Mike Titterington, New York, obituary, Royal Society, statistical methodology, University of Oxford on January 20, 2022 by xi'an**I**t is with much sadness that I heard from Oxford yesterday night that David Cox had passed away. Hither goes a giant of the field, whose contributions to theoretical and methodological statistics are enormous and whose impact on society is truly exceptional. He was the first recipient of the International Prize in Statistics in 2016 (aka the “Nobel of Statistics”) among many awards and a Fellow of the Royal Society among many other recognitions. He was also the editor of *Biometrika* for 25 years (!) and was still submitting papers to the journal a few month ago. Statistical Science published a conversation between Nancy Reid and him that tells a lot about the man and his amazing modesty. While I had met him in 1989, when he was visiting Cornell University as a distinguished visitor (and when I drove him to the house of Anne and George Casella for dinner once), then again in the 1990s when he came on a two-day visit to CREST, we only really had a significant conversation in 2011 (!), when David and I attended the colloquium in honour of Mike Titterington in Glasgow and he proved to be most interested in the ABC algorithm. He published a connected paper in *Biometrika* the year after, with Christiana Katsonaki. We met a few more times later, always in Oxford, to again discuss ABC. In each occasion, he was incredibly kind and considerate.

## more air [&q’s] for MCMC [comments]

Posted in Books, pictures, Statistics with tags convergence assessment, Cornell University, Delaunay triangulation, MCMC, q value, quasi-posterior, R factor, R-hat, Riemann sums, SAMSI on June 11, 2021 by xi'an*[A rich set of comments by Tom Loredo about convergence assessments for MCMC that I feel needs reposting:]*

Two quick points:

- By coincidence (and for a different problem), I’ve just been looking at the work of Gorham & Mackey that I believe Pierre is referring to. This is probably the relevant paper: “Measuring Sample Quality with Kernels“.
- Besides their new rank-based R-hat, bloggers on Gelman’s blog have also pointed to another R-hat replacement, R
*, developed by some Stan team members; it is “based on how well a machine learning classifier model can successfully discriminate the individual chains.” See: “R*: A robust MCMC convergence diagnostic with uncertainty using decision tree classifiers”.

In addition, here’s an anecdote regarding your comment, “I remain perplexed and frustrated by the fact that, 30 years later, the computed values of the visited likelihoods are not better exploited.”

That has long bothered me, too. During a SAMSI program around 2006, I spent time working on one approach that tried to use the prior*likelihood (I call it q(*θ*), for “quasiposterior” and because it’s next to “p”!) to compute the marginal likelihood. It would take posterior samples (from MCMC or another approach) and find their Delaunay triangulation. Then, using q(*θ*) on the nodes of the simplices comprising the triangulation, it used a simplicial cubature rule to approximate the integral of q(theta) over the volume spanned by the samples.

As I recall, I only explored it with multivariate normal and Student-t targets. It failed, but in an interesting way. It worked well in low dimensions, but gave increasingly poor estimates as dimension grew. The problem appeared related to concentration of measure (or the location of the typical set), with the points not sufficiently covering the center or the large volume in the tails (or both; I can’t remember what diagnostics said exactly).

Another problem is that Delaunay triangulation gets expensive quickly with growing dimension. This method doesn’t need an optimal triangulation, so I wondered if there was a faster sub-optimal triangulation algorithm, but I couldn’t find one.

An interesting aspect of this approach is that the fact that the points are drawn from the prior doesn’t matter. Any set of points is a valid set of points for approximating the integral (in the spanned volume). I just used posterior samples because I presumed those would be available from MCMC. I briefly did some experiments taking the samples, and reweighting them to draw a subset for the cubature that was either over- or under-dispersed vs. the target. And one could improve things this way (I can’t remember what choice was better). This suggests that points drawn from q(theta) aren’t optimal for such cubature, but I never tried looking formally for the optimal choice.

I called the approach “adaptive simplicial cubature,” adaptive in the sense that the points are chosen in a way that depends on the integrand.

The only related work I could find at the time was work by *you* and Anne Philippe on Riemanns sums with MCMC (https://doi.org/10.1023/A:1008926514119). I later stumbled upon a paper on “random Riemann sum estimators” as an alternative to Monte Carlo that seems related but that I didn’t explore further (https://doi.org/10.1016/j.csda.2006.09.041).

I still find it hard to believe that the q values aren’t useful. Admittedly, in an n-dimensional distribution, it’s just 1 more quantity available beyond the n that comprise the sample location. But it’s a qualitatively different type of information from the sample *location*, and I can’t help but think there’s some clever way to use it (besides emulating the response surface).

## estimation of a normal mean matrix

Posted in Statistics with tags Biometrika, Charles Stein, Cornell University, James-Stein estimator, Purdue University, Rutgers University, shrinkage estimation, Springer-Verlag, superharmonicity, Université de Rouen on May 13, 2021 by xi'an**A** few days ago, I noticed the paper Estimation under matrix quadratic loss and matrix superharmonicity by Takeru Matsuda and my friend Bill Strawderman had appeared in Biometrika. *(Disclaimer: I was not involved in handling the submission!)* This is a “classical” shrinkage estimation problem in that covariance matrix estimators are compared under under a quadratic loss, using Charles Stein’s technique of unbiased estimation of the risk is derived. The authors show that the Efron–Morris estimator is minimax. They also introduce superharmonicity for matrix-variate functions towards showing that generalized Bayes estimator with respect to a matrix superharmonic priors are minimax., including a generalization of Stein’s prior. Superharmonicity that relates to (much) earlier results by Ed George (1986), Mary-Ellen Bock (1988), Dominique Fourdrinier, Bill Strawderman, and Marty Wells (1998). (All of whom I worked with in the 1980’s and 1990’s! in Rouen, Purdue, and Cornell). This paper also made me realise Dominique, Bill, and Marty had published a Springer book on Shrinkage estimators a few years ago and that I had missed it..!