Archive for finite mixtures

scolded!

Posted in Books, Kids, University life with tags , , , , , , , , , on October 9, 2026 by xi'an

I recently received a request for refereeing a paper for a journal I had never heard of, in a field far, far away from my own, with an abstract that indicated little innovation beyond transferring a by now standard technique to this very field. Given the volume of such requests, I immediately declined and received the scolding and peeved reply, as of below, from said journal editor! (The above picture is from Bayesian Core, not from the paper submitted.)

Invited Reviewer:

Thank you for letting us know that you are not willing to review the submission, “Bayesian Analysis of Mixtures with an Unknown Number of Components &tc,” for Journal of Some Socio-Economics Issues®.

It is most unfortunate that you did not choose to make your impression that “the contents sound very standard given the existing literature” known to the author/s on behalf of the Journal.  It would certainly have aided them in the longer run.
We also find it rather extraordinary for a potential reviewer to turn down an invitation “because there is little possibility I [will] ever publish in your journal”.  So much for fostering your discipline and academic excellence outside of a specific circle of publications … 

Best regards,

The editor

Gaussian mixture via continuous sparse regularization

Posted in Books, Mountains, Statistics, University life with tags , , , , , , , , on December 6, 2025 by xi'an

There were several talks and posters in Aussois involving mixtures, incl. one by Romane Giard on a proposal recently posted on arXiv. The mixture is made of multivariate Gaussian components with unknown but diagonal covariance matrices. The number s of components is unknown as well. The estimation approach is based on a form of LASSO that relies on a point process representation of the mixture parameters, an approach long defended by Peter Green (as summarised in his introductory chapter for our Handbook of Mixture Analysis.  Which avoids unnecessary confusions about identifiability and label switching.

The procedure of estimation is thus (B-)LASSO, meaning a data fitting (or fidelity) term comparing a kernel density estimate (seen as a Gaussian convolution) to the smoothed predicted density. Plus a sparsity term, namely a total variation norm of the point process measure. The theoretical result requires a fixed minimum on the variances and a minimum distance between the component parameter vectors. Things get a bit more complicated (for me), given that the point process measure gets replaced by a weighted, approximate, version where each component is attributed a positive weight depending on the variance parameters uk of the Gaussians, a product of (uk²+τ²)-1/4 terms (with τ a regularising parameter). Since the paper is fully focussed on theoretical properties, it does not expand on the practical derivation of the estimator, acknowledging the numerical resolution of the minimisation problem is yet unknown, with a substitute “closer to a realistic algorithmic framework” (p20).

bridging ratio estimators

Posted in Books, Statistics, University life with tags , , , , , , , , , , on June 3, 2025 by xi'an

easily computed marginal likelihoods for multivariate mixture models using the THAMES estimator

Posted in Books, Statistics, University life with tags , , , , , , , , , , , , , , , , , , , , , on May 25, 2025 by xi'an

Martin Metodiev and his coauthor(es)s have produced another paper on the THAMES Monte Carlo method when specifically targetting marginal likelihoods for mixture models. Since this problem has long been a central interest of mine’s and since the method is closely connected with the harmonic mean solution we developed with Darren Wraith in 2009, (and also included in our 2009 survey with Jean-Michel Marin of evidence approximations, published in Frontiers of Statistical Decision Making and Bayesian Analysis for Jim Berger’s 60th birthday), I quickly went into the paper. The core purpose of this paper is to adapt THAMES to a multimodal setting since using an ellipsoidal region as the support of the Uniform reciprocal importance sampling distribution does not make sense for a multimodal target. After reading it a few times, and while some computational aspects remain obscure to me, I am not convinced this brings an adequate answer to the challenge.  Indeed, while the approach borrows directly from Berkhof et al. (2003) that inspired the resolution we proposed, Jeong (Kate) Lee and myself, the issues I have with the current proposal are that

1. the evacuation of earlier methods as not simple or not universal enough is rather disingenuous. For instance, software that do not return (latent) allocation vectors can easily be post-processed. And the current method uses allocation probabilities just the same (in Section 3.3). Similarly, the random shuffling answer to label (lack of) switching proposed by Sylvia Früwirth-Schnatter—which again can be achieved by post-processing—cannot be rejected on the sole basis that the component means (based on the MCMC sample) are all similar. It is furthermore debatable that the current proposal is simple, when involving relabelling à la Stephens, averaging over permutations, selecting over said permutations by constructing a graph over components (section 3.2.1) and  running a quadratic discriminant analysis (section 3.2.2) on the posterior sample, based on an arbitrary Normal representation of the distributions of the clusters, and finally defining a new ordering constraint (section 3.2.3). Computing efforts  required by the respective methods do not appear in the main text.

2. the handling of the label switching issue—the reason why Larry Wasserman saw mixtures at the same magnitude of evil as tequila!—is problematic for several reasons. My position (since at least 2000!) on the matter is that the proper posterior sample must exhibit label switching and come close to symmetry among the “components”. The label switching problem (section 3.1) is rather when the MCMC sample does not “switch their labels”. The relabelling approach (e.g., à la Stephens) allows for a differentiation between components, to some extent, which helps with computing basic posterior moments for point estimation or for the calibration of the support of the Uniform reciprocal importance sampling distribution, but the use of any relabelling procedure is tampering with the original MCMC sample and thus bound to impact the distribution of the resulting relabelled sample. Furthermore, relabelling depends on the value of G, whereas the actual number of (significant) modes in the posterior is also connected with the (partial) fit of the data to the model, meaning the creation of further modes than those linked with relabelling. Especially when the model is misspecified. Incidentally, the symmetrised version of THAMES (5) does not require relabelling. Neither does the Bayes factor. In addition, the experiment section (4.1.2) mentions that bridge sampling is biased by a factor of G!, which comes as a surprise to me since I associated this factor with the call to Sid Chib’s formula in the absence of label switching, i.e. when the MCMC sample was stuck on a mode, as exposed by Radford Neal in 1999. Is it because bridge sampling is applied to the relabelled sample? It is also surprising that the gap appears in the simulated datasets (Fig.3) and not in the real ones (Fig.5).

3. the (legitimate) purpose of using marginal likelihoods for selecting the number G of components is weakened by the intrusion of alternate proposals to assess G from the data, like the criterion of overlap (section 3.2.1), which instead aims at the number of clusters, with an elimination of “empty components” that should either remain a possibility (within a regular mixture model) or be evacuated with a different modelling (à la Diebolt & Robert, or à la Wasserman). This overlapping criterion is further used in the discriminant analysis that only applies to “non-overlapping components” of the mixture (section 3.2.3)—at which point I got lost in the reordering and simplification of the computation of THAMES (but got reminded of the results of Agostino Nobile in the 2000’s, with whom I used to discuss a lot in my yearly visit to the University of Glasgow).

4. several mentions are made of the other estimators being biased, which is indeed the case for bridge sampling (if not necessarily for importance sampling), but not necessarily a central issue, while the original generalised harmonic proposal by Gelfand and Dey (1994) and thus THAMES produce an unbiased estimator of the inverse of the evidence (thus neither of the evidence nor of the log-evidence). However, in the paper, the volume of the support of the Uniform reciprocal importance sampling distribution is estimated by a basic Monte Carlo coverage probability in (3), which induces the same type of bias as the other methods.

Seminal ideas and controversies in Statistics [book review]

Posted in Books, Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , on May 24, 2025 by xi'an

CRC Press sent CHANCE this book for review. Since the topic was of clear interest to me, with an author who significantly contributed to the field—my only recollection meeting Roderick Little was during the Australian Statistical Conference in Adelaïde, in 2012, at the start of my Oz 2012 Tour!—, I took the opportunity of the nearest weekend to browse through Seminal ideas and controversies in Statistics. I like very much the idea of selecting a dozen key papers in the history of Statistics and of discussing why. In fact, this reminded me of my classics seminar, which lasted the few years I was 100% in charge of the Master program in Dauphine (and which I hope I could restart!). Checking the list of the papers I then suggested my students, I see some overlap with 9 papers out of the 15 groups. (I also remember Steve Fienberg making suggestions for that list, while he was spending a sabbatical in Paris at CREST.) Given that community of focus and purpose, and contrary to my wont, I have really very little of substance to criticize or wish about the book. The less when reading the following

“On a personal note, I met Yates [author of a 1984 paper on tests for 2×2 contingency tables discussing the relevance of conditioning on one or both margins], a charming man, when I was a young graduate student who knew next to nothing about statistics; we discussed the joys of traversing the Cuillin Ridge in Skye.”

since completing that ridge remains high in my mountain-climbing bucket-list! (Possibly next year, since we are running an ICMS workshop on the Island.)

The first paper in the series is more than a foundational paper since (The) Fisher’s 1922 paper is about creating (almost) ex nihilo the field of (modern) mathematical statistics. I don’t know if there is any equivalence in other scientific disciplines of such an impact (and of such a man)… Roderick Little manages to convincingly engage with Fisher’s dismissive views on (not yet called) Bayesian analysis, although, to the latter’s defence, the formalisation of Bayesian inference at that time had not yet emerged. The second chapter is discussing Yates’ 1984 paper on tests for 2×2 contingency tables that he wrote 50 years after writing the original one in the first volume of JRSS. Roderick Little adds a detailed Bayesian analysis with the three standard reference priors, Jeffreys’ version proving quite close to Fisher’s exact test (conditional on both margins). The third chapter is aiming at the generic challenge of hypothesis testing, from the well-known opposition between Fisher and Neyman (both on the cover), to questioning the sanity of hard-set thresholds (with a mention of our American Statistician call to abandon (shi)p!). The later (thus) refers to the recent literature on the replicability crisis and the now famous ASA statement on p-values by Ron Wasserstein and Nicole Lazar, analysed in the chapter. But I would have like to read another full section on alternatives to hypothesis testing. While now a niche interest (imho), Fisher’s attempt at creating a posterior distribution without a prior, aka fiducial inference, is discussed in Chapter 4 with the Behrens-Fisher problem as the illustrating example. The chapter feels rather anticlimactic, with the comparison relying on the (Malay) Ghosh and Kim (2001) simulation results.

Birnbaum’s (1962) likelihood principle is the topic of Chapter 5 (and I cannot remember any of my students choosing this paper over the years, although there was at least one). Roderick Little recalls some sentences from the JASA discussion as an appetiser, a reminder of the time when these discussions could turn in scathing attacks. The chapter contains excerpts from Berger and Wolpert (1988)—which they were writing while I was spending a year at Purdue and which I have always recommended to my PhD students, albeit not for the classic seminar. It then moves to the controversies that surround this principle since its inception, in particular those accumulated by Deborah Mayo (also on the cover) as reported on the ‘Og. In the recent years, I have become less excited about the LP, in part due to the imprecision in its statement, which opens the door to conflicting interpretations. And in part due to the scarcity of models with non-trivial sufficient statistics. (I am also wondering if the sufficiency issue we highlighted in our ABC model choice criticism does relate to the mixture example at the end of the chapter.)

The next chapter is one all for compromise, through the calibrated Bayes perspective that credible statements should be close to confidence statements in the long run. Which I remember him presenting at ASC 2012. The concept is found in the very 1984 paper by Don Rubin (also on the cover) that contains the concept behind Approximate Bayesian Computation (ABC). And the chapter proceeds by listing strengths and weaknesses of frequentist and Bayesian perspectives, towards a fusion of both., e.g. though posterior predictive checks.

While the choice of a (general public) paper from Scientific American may sound surprising in Chapter 7, with Efron’s (on the cover) and Morris’ 1977 Stein’s paradox, I cannot but applaud, the more because this was the first paper I read when starting my PhD on the James-Stein estimators. Although this may sound like happening eons ago, the James and Stein (1961) paper—which is my age!—”created a considerable backlash” by toppling unbiasedness from its pedestal and exhibiting a paradox that 1+1+1≠3… Which Little reinterprets via a random effect (or Bayesian hierarchical) model. (And a chapter where I learned that Little’s father was a journalist, a characteristic he shared with Bruce Lindsay, as I found at Blonde, Glasgow, during an ICMS workshop). Relatedly, the next chapter is about the “57 varieties [of regression] paper” by Demptster, Schatzoff and Wermuth (1977). Apparently connected with Heinz 57 varieties of pickles. The paper considers Stein and ridge and variable selections versions for variable selection. The chapter also covers (Bayesian) Lasso and BART, as well as a brief all too brief mention of Spike & Slab priors—with my friend Veronika Ročková missing from the authors’ index!—,  but I was expecting from the title other, robust, forms of regression like L¹ regression and econometrics digressions. Chapter 10 can however been seen as a proxy since covering generalized estimating equations from a 1986 Biometrika paper of Liang and Zeger, with no Bayesian aspect (and an expected appearance of Communications in Statistics B).

Chapter 9 covers the almost immediately classic 1995 paper of Benjamini and Hochbeg on multiple regressions (that Series B turned into a discussion paper ten years later!). Although it spends more time on Berry’s (2012) recommendations than on FDR. The computational Chapter 11 brings together Efron’s (1979) bootstrap [with his picture on the cover] and MCMC, represented by the founding paper of Gelfand and Smith (1990, if mistakenly set in 1988 on p140). A bit of a strange mix imho as the former is more inferential than computational. And not giving the EM algorithm that much space. And not questioning MCMC methods as a good proxy to posterior distributions. Tukey’s Future of Data Analysis (as founding exploratory data analysis) and Breiman’s Two cultures (as launching statistical machine learning) meet in Chapter 12. (With a reminder that the latter invokes Occam’s razor—which may not be that appropriate for hugely overparameterised machine learning black boxes—and…the Rashomon principle! Meaning that distinct models may all fit the same data. Let me nitpickingly add the reference to Ryûnosuke Akutagawa as the author of Rashômon and other stories that Kurosawa adapted in his splendid movie). The chapter contains critical remarks from David Cox, Brad Efron, David Bickel, and Andrew Gelman, with a further section on Little’s view on modelling.

The last three chapters are on design and sampling, in connection with Little’s (and Rubin’s) works in the area. With a 1934 paper of Neyman (whose picture on the cover could have been chosen differently, albeit no fault of Neyman [or of Little!] that his toothbrush style of moustache dramatically got out of fashion!). With a return to calibrated Bayes and a reminiscence of Little’s time at the World Fertility Survey but (apparently) no mention of the probabilistic aspects of modern censuses (that saw my friends Steve Fienberg on the one side and Larry Brown and Marty Wells on the other side argue for and against it!), again relating to the reliance on statistical models. Chapter 14 relates randomized clinical trials to causality, which makes a (worthy) appearance there. Roderick Little also makes a clear case there against the retracted study linking vaccines and autism, a call that will unlikely not reach the current Trump administration and its Secretary of Health.

The book concludes with a list of twenty style and grammar suggestions for improved writing.

As should be crystal-clear from the above, I quite enjoyed the book and would definitely use its reading list in a graduate course whenever the opportunity arises. Once again, some choices are more personal to the author than others, and I would have place more emphasis on the fantastic Dawid, Stone and Zidek (1973)—with Jim Zidek also missing from the author index—, but all make sense in a walk through statistical classics. Let me however regret the absence therein of major actors like, e.g., D. Blackwell, C.R. Rao,  or G. Wahba (except in a stylistic example p199), two of whom were awarded the International Prize in Statistics.

[Disclaimer about potential self-plagiarism: this post or an edited version will eventually appear in my Books Review section in CHANCE.]