## ISBA 2016 [#5]

Posted in Mountains, pictures, Running, Statistics, Travel with tags , , , , , , , , , , , , , on June 18, 2016 by xi'an

On Thursday, I started the day by a rather masochist run to the nearby hills, not only because of the very hour but also because, by following rabbit trails that were not intended for my size, I ended up being scratched by thorns and bramble all over!, but also with neat views of the coast around Pula.  From there, it was all downhill [joke]. The first morning talk I attended was by Paul Fearnhead and about efficient change point estimation (which is an NP hard problem or close to). The method relies on dynamic programming [which reminded me of one of my earliest Pascal codes about optimising a dam debit]. From my spectator’s perspective, I wonder[ed] at easier models, from Lasso optimisation to spline modelling followed by testing equality between bits. Later that morning, James Scott delivered the first Bayarri Lecture, created in honour of our friend Susie who passed away between the previous ISBA meeting and this one. James gave an impressive coverage of regularisation through three complex models, with the [hopefully not degraded by my translation] message that we should [as Bayesians] focus on important parts of those models and use non-Bayesian tools like regularisation. I can understand the practical constraints for doing so, but optimisation leads us away from a Bayesian handling of inference problems, by removing the ascertainment of uncertainty…

Later in the afternoon, I took part in the Bayesian foundations session, discussing the shortcomings of the Bayes factor and suggesting the use of mixtures instead. With rebuttals from [friends in] the audience!

This session also included a talk by Victor Peña and Jim Berger analysing and answering the recent criticisms of the Likelihood principle. I am not sure this answer will convince the critics, but I won’t comment further as I now see the debate as resulting from a vague notion of inference in Birnbaum‘s expression of the principle. Jan Hannig gave another foundation talk introducing fiducial distributions (a.k.a., Fisher’s Bayesian mimicry) but failing to provide a foundational argument for replacing Bayesian modelling. (Obviously, I am definitely prejudiced in this regard.)

The last session of the day was sponsored by BayesComp and saw talks by Natesh Pillai, Pierre Jacob, and Eric Xing. Natesh talked about his paper on accelerated MCMC recently published in JASA. Which surprisingly did not get discussed here, but would definitely deserve to be! As hopefully corrected within a few days, when I recoved from conference burnout!!! Pierre Jacob presented a work we are currently completing with Chris Holmes and Lawrence Murray on modularisation, inspired from the cut problem (as exposed by Plummer at MCMski IV in Chamonix). And Eric Xing spoke about embarrassingly parallel solutions, discussed a while ago here.

## about the strong likelihood principle

Posted in Books, Statistics, University life with tags , , , , , , , on November 13, 2014 by xi'an

Deborah Mayo arXived a Statistical Science paper a few days ago, along with discussions by Jan Bjørnstad, Phil Dawid, Don Fraser, Michael Evans, Jan Hanning, R. Martin and C. Liu. I am very glad that this discussion paper came out and that it came out in Statistical Science, although I am rather surprised to find no discussion by Jim Berger or Robert Wolpert, and even though I still cannot entirely follow the deductive argument in the rejection of Birnbaum’s proof, just as in the earlier version in Error & Inference.  But I somehow do not feel like going again into a new debate about this critique of Birnbaum’s derivation. (Even though statements like the fact that the SLP “would preclude the use of sampling distributions” (p.227) would call for contradiction.)

“It is the imprecision in Birnbaum’s formulation that leads to a faulty impression of exactly what  is proved.” M. Evans

Indeed, at this stage, I fear that [for me] a more relevant issue is whether or not the debate does matter… At a logical cum foundational [and maybe cum historical] level, it makes perfect sense to uncover if and which if any of the myriad of Birnbaum’s likelihood Principles holds. [Although trying to uncover Birnbaum’s motives and positions over time may not be so relevant.] I think the paper and the discussions acknowledge that some version of the weak conditionality Principle does not imply some version of the strong likelihood Principle. With other logical implications remaining true. At a methodological level, I am less much less sure it matters. Each time I taught this notion, I got blank stares and incomprehension from my students, to the point I have now stopped altogether teaching the likelihood Principle in class. And most of my co-authors do not seem to care very much about it. At a purely mathematical level, I wonder if there even is ground for a debate since the notions involved can be defined in various imprecise ways, as pointed out by Michael Evans above and in his discussion. At a statistical level, sufficiency eventually is a strange notion in that it seems to make plenty of sense until one realises there is no interesting sufficiency outside exponential families. Just as there are very few parameter transforms for which unbiased estimators can be found. So I also spend very little time teaching and even less worrying about sufficiency. (As it happens, I taught the notion this morning!) At another and presumably more significant statistical level, what matters is information, e.g., conditioning means adding information (i.e., about which experiment has been used). While complex settings may prohibit the use of the entire information provided by the data, at a formal level there is no argument for not using the entire information, i.e. conditioning upon the entire data. (At a computational level, this is no longer true, witness ABC and similar limited information techniques. By the way, ABC demonstrates if needed why sampling distributions matter so much to Bayesian analysis.)

“Non-subjective Bayesians who (…) have to live with some violations of the likelihood principle (…) since their prior probability distributions are influenced by the sampling distribution.” D. Mayo (p.229)

In the end, the fact that the prior may depend on the form of the sampling distribution and hence does violate the likelihood Principle does not worry me so much. In most models I consider, the parameters are endogenous to those sampling distributions and do not live an ethereal existence independently from the model: they are substantiated and calibrated by the model itself, which makes the discussion about the LP rather vacuous. See, e.g., the coefficients of a linear model. In complex models, or in large datasets, it is even impossible to handle the whole data or the whole model and proxies have to be used instead, making worries about the structure of the (original) likelihood vacuous. I think we have now reached a stage of statistical inference where models are no longer accepted as ideal truth and where approximation is the hard reality, imposed by the massive amounts of data relentlessly calling for immediate processing. Hence, where the self-validation or invalidation of such approximations in terms of predictive performances is the relevant issue. Provided we can at all face the challenge…

## who’s afraid of the big B wolf?

Posted in Books, Statistics, University life with tags , , , , , , , , , , on March 13, 2013 by xi'an

Aris Spanos just published a paper entitled “Who should be afraid of the Jeffreys-Lindley paradox?” in the journal Philosophy of Science. This piece is a continuation of the debate about frequentist versus llikelihoodist versus Bayesian (should it be Bayesianist?! or Laplacist?!) testing approaches, exposed in Mayo and Spanos’ Error and Inference, and discussed in several posts of the ‘Og. I started reading the paper in conjunction with a paper I am currently writing for a special volume in  honour of Dennis Lindley, paper that I will discuss later on the ‘Og…

“…the postdata severity evaluation (…) addresses the key problem with Fisherian p-values in the sense that the severity evaluation provides the “magnitude” of the warranted discrepancy from the null by taking into account the generic capacity of the test (that includes n) in question as it relates to the observed data”(p.88)

First, the antagonistic style of the paper is reminding me of Spanos’ previous works in that it relies on repeated value judgements (such as “Bayesian charge”, “blatant misinterpretation”, “Bayesian allegations that have undermined the credibility of frequentist statistics”, “both approaches are far from immune to fallacious interpretations”, “only crude rules of thumbs”, &tc.) and rhetorical sleights of hand. (See, e.g., “In contrast, the severity account ensures learning from data by employing trustworthy evidence (…), the reliability of evidence being calibrated in terms of the relevant error probabilities” [my stress].) Connectedly, Spanos often resorts to an unusual [at least for statisticians] vocabulary that amounts to newspeak. Here are some illustrations: “summoning the generic capacity of the test”, ‘substantively significant”, “custom tailoring the generic capacity of the test”, “the fallacy of acceptance”, “the relevance of the generic capacity of the particular test”, yes the term “generic capacity” is occurring there with a truly high frequency. Continue reading

Posted in Statistics with tags , , , , , , , , , on January 28, 2013 by xi'an

Last Monday, my student Li Chenlu presented the foundational 1962 JASA paper by Allan Birnbaum, On the Foundations of Statistical Inference. The very paper that derives the Likelihood Principle from the cumulated Conditional and Sufficiency principles and that had been discussed [maybe ad nauseam] on this ‘Og!!! Alas, thrice alas!, I was still stuck in the plane flying back from Atlanta as she was presenting her understanding of the paper, as the flight had been delayed four hours thanks to (or rather woe to!) the weather conditions in Paris the day before (chain reaction…):

I am sorry I could not attend this lecture and this for many reasons: first and  foremost, I wanted to attend every talk from my students both out of respect for them and to draw a comparison between their performances. My PhD student Sofia ran the seminar that day in my stead, for which I am quite grateful, but I do do wish I had been there… Second, this a.s. has been the most philosophical paper in the series.and I would have appreciated giving the proper light on the reasons for and the consequences of this paper as Li Chenlu stuck very much on the paper itself. (She provided additional references in the conclusion but they did not seem to impact the slides.)  Discussing for instance Berger’s and Wolpert’s (1988) new lights on the topic, as well as Deborah Mayo‘s (2010) attacks, and even Chang‘s (2012) misunderstandings, would have clearly helped the students.

## the likelihood principle (sequel)

Posted in Statistics with tags , , , , , on November 30, 2012 by xi'an

As mentioned in my review of Paradoxes in Scientific Inference I was a bit confused by this presentation of the likelihood principle and this led me to ponder for a week or so whether or not there was an issue with Birnbaum’s proof (or, much more likely, with my vision of it!). After reading again Birnbaum’s proof, while sitting down in a quiet room at ICERM for a little while, I do not see any reason to doubt it. (Keep reading at your own risk!)

My confusion was caused by mixing sufficiency in the sense of Birnbaum’s mixed experiment with sufficiency in the sense of our ABC model choice PNAS paper, namely that sufficient statistics are not always sufficient to select the right model. The sufficient statistics in the proof reduces the (2,x2) observation from Model 2 to (1,x1) from Model 1 when there is an observation x1 that produces a likelihood proportional to the likelihood for x2 and the statistic is indeed sufficient: the distribution of (2,x2) given (1,x1) does not depend on the parameter θ. Of course, the statistic is not sufficient (most of the time) for deciding between Model 1 and Model 2, but this model choice issue is foreign to Birnbaum’s construction.

## That the likelihood principle does not hold…

Posted in Statistics, University life with tags , , , , , , , , , , on October 6, 2011 by xi'an

Coming to Section III in Chapter Seven of Error and Inference, written by Deborah Mayo, I discovered that she considers that the likelihood principle does not hold (at least as a logical consequence of the combination of the sufficiency and of the conditionality principles), thus that  Allan Birnbaum was wrong…. As well as the dozens of people working on the likelihood principle after him! Including Jim Berger and Robert Wolpert [whose book sells for \$214 on amazon!, I hope the authors get a hefty chunk of that ripper!!! Esp. when it is available for free on project Euclid…] I had not heard of  (nor seen) this argument previously, even though it has apparently created enough of a bit of a stir around the likelihood principle page on Wikipedia. It does not seem the result is published anywhere but in the book, and I doubt it would get past a review process in a statistics journal. [Judging from a serious conversation in Zürich this morning, I may however be wrong!]

The core of Birnbaum’s proof is relatively simple: given two experiments and about the same parameter θ with different sampling distributions and , such that there exists a pair of outcomes (y¹,y²) from those experiments with proportional likelihoods, i.e. as a function of θ

$f^1(y^1|\theta) = c f^2(y^2|\theta),$

one considers the mixture experiment E⁰ where  and are each chosen with probability ½. Then it is possible to build a sufficient statistic T that is equal to the data (j,x), except when j=2 and x=y², in which case T(j,x)=(1,y¹). This statistic is sufficient since the distribution of (j,x) given T(j,x) is either a Dirac mass or a distribution on {(1,y¹),(2,y²)} that only depends on c. Thus it does not depend on the parameter θ. According to the weak conditionality principle, statistical evidence, meaning the whole range of inferences possible on θ and being denoted by Ev(E,z), should satisfy

$Ev(E^0, (j,x)) = Ev(E^j,x)$

Because the sufficiency principle states that

$Ev(E^0, (j,x)) = Ev(E^0,T(j,x))$

this leads to the likelihood principle

$Ev(E^1,y^1)=Ev(E^0, (j,y^j)) = Ev(E^2,y^2)$

(See, e.g., The Bayesian Choice, pp. 18-29.) Now, Mayo argues this is wrong because

“The inference from the outcome (Ej,yj) computed using the sampling distribution of [the mixed experiment] E⁰ is appropriately identified with an inference from outcome yj based on the sampling distribution of Ej, which is clearly false.” (p.310)

This sounds to me like a direct rejection of the conditionality principle, so I do not understand the point. (A formal rendering in Section 5 using the logic formalism of A’s and Not-A’s reinforces my feeling that the conditionality principle is the one criticised and misunderstood.) If Mayo’s frequentist stance leads her to take the sampling distribution into account at all times, this is fine within her framework. But I do not see how this argument contributes to invalidate Birnbaum’s proof. The following and last sentence of the argument may bring some light on the reason why Mayo considers it does:

“The sampling distribution to arrive at Ev(E⁰,(j,yj)) would be the convex combination averaged over the two ways that yj could have occurred. This differs from the  sampling distributions of both Ev(E1,y1) and Ev(E2,y2).” (p.310)

Indeed, and rather obviously, the sampling distribution of the evidence Ev(E*,z*) will differ depending on the experiment. But this is not what is stated by the likelihood principle, which is that the inference itself should be the same for and . Not the distribution of this inference. This confusion between inference and its assessment is reproduced in the “Explicit Counterexample” section, where p-values are computed and found to differ for various conditional versions of a mixed experiment. Again, not a reason for invalidating the likelihood principle. So, in the end, I remain fully unconvinced by this demonstration that Birnbaum was wrong. (If in a bystander’s agreement with the fact that frequentist inference can be built conditional on ancillary statistics.)

## IMS Lecture Notes on line

Posted in Books, Statistics with tags , , on April 28, 2010 by xi'an

When writing the review of Sober’s Evidence and Evolution: The Logic Behind the Science, I incidentaly found that all IMS Lecture Notes books are available on-line, free, through Project euclid. This is fantastic! This is for instance the case for Berger and Wolpert’s classic, The Likelihood Principle.