## ICM 2018

Posted in pictures, Statistics, Travel, University life with tags , , , , , , , on August 4, 2018 by xi'an

While I am not following the International Congress of Mathematicians which just started in Rio, and even less attending, I noticed an entry on their webpage on my friend and colleague Maria Esteban which I would have liked to repost verbatim but cannot figure how. (ICM 2018 also features a plenary lecture by Michael Jordan on gradient based optimisation [which was also Michael’s topic at ISBA 2018] and another one by Sanjeev Arora on the maths deep learning, two talks broadly related with statistics, which is presumably a première at this highly selective maths conference!)

## X entropy for optimisation

Posted in Books, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on March 29, 2018 by xi'an

At Gregynog, with mounds of snow still visible in the surrounding hills, not to be confused with the many sheep dotting the fields(!), Owen Jones gave a three hour lecture on simulation for optimisation, which is a less travelled path when compared with simulation for integration. His second lecture covered cross entropy for optimisation purposes. (I had forgotten that Reuven Rubinstein and Dirk Kroese had put forward this aspect of their technique in the very title of their book. As “A Unified Approach to Combinatorial Optimization, Monte-Carlo Simulation and Machine Learning”.) The X entropy approaches pushes for simulations restricted to top values of the target function, iterating to find the best parameter in the parametric family used for the simulation. (Best to be understood in the Kullback sense.) Now, this is a wee bit like simulated annealing, where lots of artificial entities have to be calibrated in the algorithm, due to the original problem being unrelated to an specific stochastic framework. X entropy facilitates concentration on the highest values of the target, but requires a family of probability distributions that puts weight on the top region. This may be a damning issue in large dimensions. Owen illustrated the approach in the case of the travelling salesman problem, where the parameterised distribution is a Markov chain on the state space of city sequences. Further, if the optimal value of the target is unknown, avoiding getting stuck in a local optimum may be tricky. (Owen presented a proof of convergence for a temperature going to zero slowly enough that is equivalent to a sure exploration of the entire state space, in a discrete setting, which does not provide a reassurance in this respect, as the corresponding algorithm cannot be implemented.) This method falls into the range of methods that are doubly stochastic in that they rely on Monte Carlo approximations at each iteration of the exploration algorithm.

During a later talk, I tried to recycle one of my earlier R codes on simulated annealing for sudokus, but could not find a useful family of proposal distributions to reach the (unique) solution. Using a mere product of distributions on each of the free positions in the sudoku grid only led me to a penalty of 13 errors…

1    2    8    5    9    7    4    9    3
7    3    5    1    2    4    6    2    8
4    6    9    6    3    8    5    7    1
2    7    5    3    1    6    9    4    8
8    1    4    7    8    9    7    6    2
6    9    3    8    4    2    1    3    5
3    8    6    4    7    5    2    1    9
1    4    2    9    6    3    8    5    7
9    5    7    2    1    8    3    4    6


It is hard to consider a distribution on the space of permutations, 𝔖⁸¹.

## journée algorithmes stochastiques à Dauphine vendredi

Posted in Statistics, University life with tags , , , , , , , , , , on November 28, 2017 by xi'an

A final reminder (?) that we hold a special day of five talks around stochastic algorithms at Dauphine this Friday, from 10:00 till 17:30. Attendance is free, coffee and tea are free (while they last!), come and join us!

## SAME but different

Posted in Statistics, University life with tags , , , , , , , , , on October 27, 2014 by xi'an

After several clones of our SAME algorithm appeared in the literature, it is rather fun to see another paper acknowledging the connection. SAME but different was arXived today by Zhao, Jiang and Canny. The point of this short paper is to show that the parallel implementation of SAME leads to efficient performances compared with existing standards. Since the duplicated latent variables are independent [given θ] they can be simulated in parallel. They further assume independence between the components of those latent variables. And finite support. As in document analysis. So they can sample the replicated latent variables all at once. Parallelism is thus used solely for the components of the latent variable(s). SAME is normally associated with an annealing schedule but the authors could not detect an improvement over a fixed and large number of replications. They reported gains comparable to state-of-the-art variational Bayes on two large datasets. Quite fun to see SAME getting a new life thanks to computer scientists!

## threshold schedules for ABC-SMC

Posted in Statistics, University life with tags , , , , , , , , , on October 17, 2012 by xi'an

Daniel Silk, Sarah Filippi and Michael Stumpf just posted on arXiv a new paper on ABC-SMC. They attack therein a typical problem with SMC (and PMC, and even MCMC!) methods, namely the ability to miss (global) modes of the target due to a poor initial exploration. So, if a proposal is built on previous simulations and if  those simulations have missed an important mode, it is quite likely that this proposal will concentrate on other parts of the space and miss the important mode even more. This is also why simulated annealing and other stochastic optimisation algorithms are so fickle: a wrong parameterisation (like the temperature schedule) induces the sequence to converge to a local optimum rather than to the global optimum. Since sequential ABC is a form of simulated annealing, the decreasing tolerance (or threshold) playing the part of the temperature, it is no surprise that it suffers from this generic disease…

The method proposed in the paper aims at avoiding this difficulty by looking at sudden drops in the acceptance rate curve (as a function of the tolerance ε),

$\aleph_t(\epsilon)=\int p_t(x)\mathbb{I}(\Delta(x,x^\star)\le \epsilon)\,\text{d}x,$

suggesting for threshold the value of ε that maximises the second derivative of this curve. Now, before getting to (at?) the issue of turning this principle into a practical algorithm, let me linger at the motivation for it:

“To see this, one can imagine an ε-ball expanding about the true data; at first the ball only encompasses a small number of particles that were drawn from very close to the global maximum, corresponding to the low gradient at the foot of the shape. Once ε is large enough we are able to accept the relatively large number of particles sitting in the local maximum, which causes the increase in gradient.” (p.5)

Thus, the argument for looking at values of ε preceding fast increases in the acceptance rate ℵ is that we are thus avoiding flatter and lower regions of the posterior support corresponding to a local maximum. It clearly encompasses the case studied by the authors of a global highly concentrated global mode, surrounded by flatter lower modes, but it seems to me that this is not necessarily the only possible reason for changes in the shape of the acceptance probability ℵ. First, we are looking at an ABC acceptance rate, not at a genuine likelihood. Second, this acceptance rate function depends on the current (and necessarily rough) approximate to the genuine predictive, pt. Furthermore, when taking into account this rudimentary replacement of the true likelihood function, it is rather difficult to envision how it impacts the correspondence between a proximity in the data and a proximity in the parameter space. (The toy example is both illuminating and misleading, because it considers a setting where the data is a deterministic transform of the parameter.) I thus think the analysis should refer more strongly to the non-parametric literature and in particular to the k-nearest-neighbour approach recently reformulated by Biau et al.: there is no reason to push the tolerance ε all the way down to zero as this limit does not correspond to the optimal value of the tolerance. If one does not use a non-parametric determination of the “right” tolerance, the lingering question is when and why stopping the sequence of ABC simulations.

The acceptance rate function ℵ is approximated using an unscented transform. I understand neither the implementation of, nor the motivations for, this choice. Given that the function ℵ is a one-dimensional transform of the tolerance and that it actually corresponds to the cdf of the distance between the true data and the (current) pseudo-data, why couldn’t we use smooth versions of a cdf estimate based on the simulated distances, given that these distances are already computed..?  I must be missing something.

## On-line EM

Posted in Statistics, University life with tags , , , , on March 4, 2011 by xi'an

Just attended a local Big’MC seminar where Olivier Cappé gave us the ideas behind the online EM algorithm he developed with Eric Moulines. The method mixes the integrated EM technique we used in the population Monte Carlo paper with Robbin-Monro, to end up with a converging sequence with an optimal speed. The paper appeared in JRSS Series B in 2009, so I cannot say this was a complete surprise. The less because this is also the theme of the chapter Olivier wrote for the mixture book. (Soon to be ready!)

## Hill climbing

Posted in Statistics, University life with tags , , , on October 7, 2010 by xi'an

Yesterday, I received the following email from Rob Taylor:

Dr. Robert, I’ve made an observation about a variation on the Gibbs sampler that hopefully would interest you enough to want to answer my question. I’ve noticed that if I want to simply estimate the mean of a unimodal posterior density (such as a multivariate Gaussian), I can modify the Gibbs sampler to just sample the MEAN of the full conditionals at each update and get convergence to the true posterior mean in many cases. In other words I’m only sampling the posterior mean instead of sampling the target posterior distribution (or something of that flavor). So my question is: Does modifying the Gibb’s sampler to sample only the mean of the full conditionals (instead of the sampling the distribution) have any supporting theory or prior art? Empirically it seems to work very well, but I don’t know if there’s an argument for why it works.

To which I replied:  What you are implementing is closer to the EM algorithm than to Gibbs sampling. By using the (conditional) mean (or, better, mode) in unimodal conditional posteriors you are using a local maximum in one direction corresponding to the conditioned parameter and by repeating this across all parameters the algorithm increases the corresponding value of the posterior in well-behaved models. So this is a special case of hill climbing algorithm. The theory behind it is however gradient-like rather than Gibbs-like, because by taking the mean at each step you remove the randomness of a Gibbs sampler step and hence its Markovian validation. Simulated annealing would be a stochastic version of this algorithm, using Markov simulation but progressively concentrating the conditional distributions around their mode.