## a statistic with consequences

Posted in pictures, Statistics with tags , , , , , , , on July 18, 2019 by xi'an

In the latest Significance, there was a flyer with some members updates, an important one being that Sylvia Richardson had been elected the next president of the Royal Statistical Society. Congratulations to my friend Sylvia! Another item was that the publication of the 2018 RSS Statistic of the Year has led an Australian water company to switch from plastic to aluminum. Hmm, what about switching to nothing and supporting a use-your-own bottle approach? While it is correct that aluminum cans can be 100% made of recycled aluminum, this water company does not seem to appear to make any concerted effort to ensure its can are made of recycled aluminum or to increase the recycling rate for aluminum in Australia towards achieving those of Brazil (92%) or Japan (86%). (Another shocking statistic that could have been added to the 90.5% non-recycled plastic waste [in the World?] is that a water bottle consumes the equivalent of one-fourth of its contents in oil to produce.) Another US water company still promotes water bottles as one of the most effective and inert carbon capture & sequestration methods”..! There is no boundary for green-washing.

## truncated Normal moments

Posted in Books, Kids, Statistics with tags , , , , , on May 24, 2019 by xi'an

An interesting if presumably hopeless question spotted on X validated: a lower-truncated Normal distribution is parameterised by its location, scale, and truncation values, μ, σ, and α. There exist formulas to derive the mean and variance of the resulting distribution,  that is, when α=0,

$\Bbb{E}_{\mu,\sigma}[X]= \mu + \frac{\varphi(\mu/\sigma)}{1-\Phi(-\mu/\sigma)}\sigma$

and

$\text{var}_{\mu,\sigma}(X)=\sigma^2\left[1-\frac{\mu\varphi(\mu/\sigma)/\sigma}{1-\Phi(-\mu/\sigma)} -\left(\frac{\varphi(\mu/\sigma)}{1-\Phi(-\mu/\sigma)}\right)^2\right]$

but there is no easy way to choose (μ, σ) from these two quantities. Beyond numerical resolution of both equations. One of the issues is that ( μ, σ) is not a location-scale parameter for the truncated Normal distribution when α is fixed.

## robust Bayesian synthetic likelihood

Posted in Statistics with tags , , , , , , , , , , , , , on May 16, 2019 by xi'an

David Frazier (Monash University) and Chris Drovandi (QUT) have recently come up with a robustness study of Bayesian synthetic likelihood that somehow mirrors our own work with David. In a sense, Bayesian synthetic likelihood is definitely misspecified from the start in assuming a Normal distribution on the summary statistics. When the data generating process is misspecified, even were the Normal distribution the “true” model or an appropriately converging pseudo-likelihood, the simulation based evaluation of the first two moments of the Normal is biased. Of course, for a choice of a summary statistic with limited information, the model can still be weakly compatible with the data in that there exists a pseudo-true value of the parameter θ⁰ for which the synthetic mean μ(θ⁰) is the mean of the statistics. (Sorry if this explanation of mine sounds unclear!) Or rather the Monte Carlo estimate of μ(θ⁰) coincidences with that mean.The same Normal toy example as in our paper leads to very poor performances in the MCMC exploration of the (unsympathetic) synthetic target. The robustification of the approach as proposed in the paper is to bring in an extra parameter to correct for the bias in the mean, using an additional Laplace prior on the bias to aim at sparsity. Or the same for the variance matrix towards inflating it. This over-parameterisation of the model obviously avoids the MCMC to get stuck (when implementing a random walk Metropolis with the target as a scale).

## auxiliary likelihood ABC in print

Posted in Statistics with tags , , , , , , , , on March 1, 2019 by xi'an

Our paper with Gael Martin, Brendan McCabe , David Frazier and Worapree Maneesoonthorn, with full title Auxiliary Likelihood-Based Approximate Bayesian Computation in State Space Models, has now appeared in JCGS. To think that it started in Rimini in 2009, when I met Gael for the first time at the Rimini Bayesian Econometrics conference, although we really started working on the paper in 2012 when I visited Monash makes me realise the enormous investment we made in this paper, especially by Gael whose stamina and enthusiasm never cease to amaze me!

Posted in Books, pictures, Statistics with tags , , , , , , , , , , on January 28, 2019 by xi'an

An interesting paper came out on arXiv in early December, written by Michael Brand from Monash. It is about risk-adverse Bayes estimators, which are defined as avoiding the use of loss functions (although why avoiding loss functions is not made very clear in the paper). Close to MAP estimates, they bypass the dependence of said MAPs on parameterisation by maximising instead π(θ|x)/√I(θ), which is invariant by reparameterisation if not by a change of dominating measure. This form of MAP estimate is called the Wallace-Freeman (1987) estimator [of which I never heard].

The formal definition of a risk-adverse estimator is still based on a loss function in order to produce a proper version of the probability to be “wrong” in a continuous environment. The difference between estimator and true value θ, as expressed by the loss, is enlarged by a scale factor k pushed to infinity. Meaning that differences not in the immediate neighbourhood of zero are not relevant. In the case of a countable parameter space, this is essentially producing the MAP estimator. In the continuous case, for “well-defined” and “well-behaved” loss functions and estimators and density, including an invariance to parameterisation as in my own intrinsic losses of old!, which the author calls likelihood-based loss function,  mentioning f-divergences, the resulting estimator(s) is a Wallace-Freeman estimator (of which there may be several). I did not get very deep into the study of the convergence proof, which seems to borrow more from real analysis à la Rudin than from functional analysis or measure theory, but keep returning to the apparent dependence of the notion on the dominating measure, which bothers me.

## freedom to discriminate???

Posted in Statistics with tags , , , , , , on November 18, 2018 by xi'an

“Gay students and teachers could be rejected by religious schools under changes to anti-discrimination laws being recommended by a federal review into religious freedom.” The Guardian, 9 Oct. 2018

The quote is not speaking of one of the 72 countries in the World where homosexuality is considered a crime (with 13 states applying the death penalty), but of Australia, ranked 8th on the Economist 2017 Democracy Index, where religious freedom arguments are legally recognised as a right to discriminate against homosexual students and staff. (As an aside, Australia still has a blasphemy law.)

“While the panel accepted the right of religious school to discriminate against students on the basis of gender identity or sexual orientation, it could see no justification for a school to discriminate on the basis of race, disability, pregnancy or intersex status.” The Sydney Morning Herald, 9 Oct. 2018

I find it flabbergasting that such newspeak inversions (also found in the French “Manif pour tous” slogans turning égalité into a discrimination argument against homosexual weddings and adoptions) can find their way into a legislative text. And more generally that religions can still continue to promote gender discrimination with no consequences.

## down-under ABC paper accepted in JCGS!

Posted in Books, pictures, Statistics, University life with tags , , , , , , , , , , , , on October 25, 2018 by xi'an

Great news!, the ABC paper we had originally started in 2012 in Melbourne with Gael Martin and Brendan MacCabe, before joining forces with David Frazier and Worapree Maneesoothorn, in expanding its scope to using auxiliary likelihoods to run ABC in state-space models, just got accepted in the Journal of Computational and Graphical Statistics. A reason to celebrate with a Mornington Peninsula Pinot Gris wine next time I visit Monash!