Archive for conditional sufficiency

inference with insufficient statistics #2

Posted in Books, Kids, Statistics with tags , , , , , , , on December 28, 2023 by xi'an

Another X validated question of some interest: how to infer about the parameters of a model when only given a fraction 1-α of the order statistics. For instance, the (1-α)n largest observations. On a primary level, the answer is somewhat obvious since the joint density of these observations is available in closed form. At another level, it brings out the fact that the distribution of the unobserved part of the sample given the observed one only depends on the smallest observed order statistic ς (which is thus sufficient in that sense) and ends up being the original distribution truncated at ς, which allows for a closed form EM implementation. Which is also interesting given that the moments of a Normal order statistic are not available in closed form. This reminded me of the insufficient Gibbs paper we wrote with Antoine and Robin a few months ago, except for the available likelihood. And provided fodder for the final exam of my introductory mathematical statistics course at Paris Dauphine.

simplified Bayesian analysis

Posted in Statistics with tags , , , , , , , , , , , , on February 10, 2021 by xi'an

A colleague from Dauphine sent me a paper by Carlo Graziani on a Bayesian analysis of vaccine efficiency, asking for my opinion. The Bayesian side is quite simple: given two Poisson observations, N~P(μ) and M~P(ν), there exists a reparameterisation of (μ,ν) into

e=1-μ/rν  and  λ=ν(1+(1-e)r)=μ+ν

vaccine efficiency and expectation of N+M, respectively, when r is the vaccine-to-placebo ratio of person-times at risk, ie the ratio of the numbers of participants in each group. Reparameterisation such that the likelihood factorises into a function of e and a function of λ. Using a product prior for this parameterisation leads to a posterior on e times a posterior on λ. This is a nice remark, which may have been made earlier (as for instance another approach to infer about e while treating λ as a nuisance parameter is to condition on N+M). The paper then proposes as an application of this remark an analysis of the results of three SARS-Cov-2 vaccines, meaning using the pairs (N,M) for each vaccine and deriving credible intervals, which sounds more like an exercise in basic Bayesian inference than a fundamental step in assessing the efficiency of the vaccines…

ABC in Clermont-Ferrand

Posted in Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on September 20, 2019 by xi'an

Today I am taking part in a one-day workshop at the Université of Clermont Auvergne on ABC. With applications to cosmostatistics, along with Martin Kilbinger [with whom I worked on PMC schemes], Florent Leclerc and Grégoire Aufort. This should prove a most exciting day! (With not enough time to run up Puy de Dôme in the morning, though.)