## estimation exam [best of]

Posted in Books, Kids, Statistics with tags , , , , , , , , on January 29, 2019 by xi'an

Yesterday, I received a few copies of our CRC Press Handbook of Mixture Analysis, while grading my mathematical statistics exam 160 copies. Among the few goodies, I noticed the always popular magical equality

E[1/T]=1/E[T]

that must have been used in so many homeworks and exam handouts by now that it should become a folk theorem. More innovative is the argument that E[1/min{X¹,X²,…}] does not exist for iid U(0,θ) because it is the minimum and thus is the only one among the order statistics with the ability to touch zero. Another universal shortcut was the completeness conclusion that when the integral

$\int_0^\theta \varphi(x) x^k \text{d}x$

was zero for all θ’s then φ had to be equal to zero with no further argument (only one student thought to take the derivative). Plus a growing inability in the cohort to differentiate even simple functions… (At least, most students got the bootstrap right, as exemplified by their R code.) And three stars to the student who thought of completely gluing his anonymisation tag, on every one of his five sheets!, making identification indeed impossible, except by elimination of the 159 other names.

## a book and three chapters on ABC

Posted in Statistics with tags , , , , , , , , , , on January 9, 2019 by xi'an

In connection with our handbook on mixtures being published, here are three chapters I contributed to from the Handbook of ABC, edited by Scott Sisson, Yanan Fan, and Mark Beaumont:

6. Likelihood-free Model Choice, by J.-M. Marin, P. Pudlo, A. Estoup and C.P. Robert

12. Approximating the Likelihood in ABC, by  C. C. Drovandi, C. Grazian, K. Mengersen and C.P. Robert

17. Application of ABC to Infer about the Genetic History of Pygmy Hunter-Gatherers Populations from Western Central Africa, by A. Estoup, P. Verdu, J.-M. Marin, C. Robert, A. Dehne-Garcia, J.-M. Cornuet and P. Pudlo

## a book and two chapters on mixtures

Posted in Books, Statistics, University life with tags , , , , , , , , , , , , , on January 8, 2019 by xi'an

The Handbook of Mixture Analysis is now out! After a few years of planning, contacts, meetings, discussions about notations, interactions with authors, further interactions with late authors, repeating editing towards homogenisation, and a final professional edit last summer, this collection of nineteen chapters involved thirty-five contributors. I am grateful to all participants to this piece of work, especially to Sylvia Früwirth-Schnatter for being a driving force in the project and for achieving a much higher degree of homogeneity in the book than I expected. I would also like to thank Rob Calver and Lara Spieker of CRC Press for their boundless patience through the many missed deadlines and their overall support.

Two chapters which I co-authored are now available as arXived documents:

along other chapters

## ABC in print

Posted in Books, pictures, Statistics, University life with tags , , , , , , , , , on September 5, 2018 by xi'an

The CRC Press Handbook of ABC is now out, after a rather long delay [the first version of our model choice chapter was written in 2015!] due to some late contributors Which is why I did not spot it at JSM 2018. As announced a few weeks ago, our Handbook of Mixture Analysis is soon to be published as well. (Not that I necessarily advocate the individual purchase of these costly volumes!, especially given most chapters are available on-line.)

## Handbook of Mixture Analysis [cover]

Posted in Books, Statistics, University life with tags , , , , , , , , on August 15, 2018 by xi'an

On the occasion of my talk at JSM2018, CRC Press sent me the cover of our incoming handbook on mixture analysis, courtesy of Rob Calver who managed to get it to me on very short notice! We are about ready to send the manuscript to CRC Press and hopefully the volume will get published pretty soon. It would have been better to have it ready for JSM2018, but we editors got delayed by a few months for the usual reasons.

## JSM 2018 [#4½]

Posted in Statistics, University life with tags , , , , , , , , on August 10, 2018 by xi'an

As I wrote my previous blog entry on JSM2018 before the sessions, I did not have the chance to comment on our mixture session, which I found most interesting!, with new entries on the topic and a great discussion by Bettina Grün. Including the important call for linking weights with the other parameters, as both groups being independent does not make sense when the number of components is uncertain. (Incidentally our paper with Kaniav kamary and Kate Lee does create a dependence.) The talk by Deborah Kunkel was about anchored mixture estimation, a joint work with Mario Peruggia, another arXival that I had missed.

The notion of anchoring found in this paper is to allocate specific observations to specific components. These observations are thus anchored to these components. Among other things, this modification of the sampling model implies a removal of the unidentifiability problem. Hence formally of the label-switching or lack thereof issue. (Although, as Peter Green repeatedly mentioned, visualising the parameter space as a point process eliminates the issue.) This idea is somewhat connected with the constraint Jean Diebolt and I imposed in our 1990 mixture paper, namely that no component would have less than two observations allocated to it, but imposing which ones are which of course reduces drastically the complexity of the model. Another (related) aspect of anchoring is that the observations that are anchored to the components act as parts of the prior model, modifying the initial priors (which can then become improper as in our 1990 paper). The difficulty of the anchoring approach is to find observations to anchor in an unsupervised setting. The paper proceeds by optimising the allocations, which somewhat turns the prior into a data-dependent prior since all observations are used to set the anchors and then used again for the standard Bayesian processing. In that respect, I would rather follow the sequential procedure developed by Nicolas Chopin and Florian Pelgrin, where the number of components grows by steps with the number of observations.

## LaTeX issues from Vienna

Posted in Books, Statistics, University life with tags , , , , , , , , , , , on September 21, 2017 by xi'an

When working on the final stage of our edited handbook on mixtures, in Vienna, I came across unexpected practical difficulties! One was that by working on Dropbox with Windows users, files and directories names suddenly switched from upper case to lower cases letters !, making hard-wired paths to figures and subsections void in the numerous LaTeX files used for the book. And forcing us to change to lower cases everywhere. Having not worked under Windows since George Casella gave me my first laptop in the mid 90’s!, I am amazed that this inability to handle both upper and lower names is still an issue. And that Dropbox replicates it. (And that some people see that as a plus.)

The other LaTeX issue that took a while to solve was that we opted for one chapter one bibliography, rather than having a single bibliography at the end of the book, mainly because CRC Press asked for this feature in order to sell chapters individually… This was my first encounter with this issue and I found the solutions to produce individual bibliographies incredibly heavy handed, whether through chapterbib or bibunits, since one has to bibtex one .aux file for each chapter. Even with a one line bash command,

for f in bu*aux; do bibtex basename \$f .aux; done

this is annoying in the extreme!