## dynamic mixtures [at NBBC15]

Posted in R, Statistics with tags , , , , , , , , , , , , on June 18, 2015 by xi'an

A funny coincidence: as I was sitting next to Arnoldo Frigessi at the NBBC15 conference, I came upon a new question on Cross Validated about a dynamic mixture model he had developed in 2002 with Olga Haug and Håvård Rue [whom I also saw last week in Valencià]. The dynamic mixture model they proposed replaces the standard weights in the mixture with cumulative distribution functions, hence the term dynamic. Here is the version used in their paper (x>0)

$(1-w_{\mu,\tau}(x))f_{\beta,\lambda}(x)+w_{\mu,\tau}(x)g_{\epsilon,\sigma}(x)$

where f is a Weibull density, g a generalised Pareto density, and w is the cdf of a Cauchy distribution [all distributions being endowed with standard parameters]. While the above object is not a mixture of a generalised Pareto and of a Weibull distributions (instead, it is a mixture of two non-standard distributions with unknown weights), it is close to the Weibull when x is near zero and ends up with the Pareto tail (when x is large). The question was about simulating from this distribution and, while an answer was in the paper, I replied on Cross Validated with an alternative accept-reject proposal and with a somewhat (if mildly) non-standard MCMC implementation enjoying a much higher acceptance rate and the same fit.

## capture mark recapture with no mark and no recapture [aka 23andmyfish]

Posted in Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , on June 11, 2015 by xi'an

A very exciting talk today at NBBC15 here in Reykjavik was delivered by Mark Bravington yesterday on Close-kin mark recapture by modern magic (!). Although Mark is from Australia, being a Hobart resident does qualify him for the Nordic branch of the conference! The exciting idea is to use genetic markers to link catches in a (fish) population as being related as parent-offspring or as siblings. This sounds like science-fantasy when you first hear of it!, but it is actually working better than standard capture-mark-recapture methods for populations of a certain size (so that the chances to find related animals are not the absolute zero!, as, e.g., krill populations). The talk was focussed on bluefin tuna, whose survival is unlikely under the current fishing pressure… Among the advantages, a much more limited impact of the capture on the animal, since only a small amount of genetic material is needed, no tag loss, tag destruction by hunters, or tag impact of the animal survival, no recapture, a unique identification of each animal, and the potential for a detailed amount of information through the genetic record. Ideally, the entire sample could lead to a reconstruction of its genealogy all the way to the common ancestor, a wee bit like what 23andme proposes for humans, but this remains at the science-fantasy level given what is currently know about the fish species genomes.

## visiting deCODE [Íslensk erfðagreining]

Posted in Books, Kids, pictures, Statistics, Travel, University life with tags , , , , on June 9, 2015 by xi'an

Tonight, we [participants to the NBBC15 conference] got invited [and bused] to deCODE, the Icelandic genetic company that has worked on the human genome since 1996, taking advantage of the uniquely homogeneous features of the Icelandic population. Which overwhelmingly descends from the few originals settlers to populate Iceland in the late 800’s.  deCODE is located in downtown Reyjavik next to the regional airport and to Hallgrímskirkja, the iconic church overlooking the city. The genetic company has gathered genotypic and phenotypic information about half the population of Iceland and, thanks to extensive genealogical sources, has also put together the Íslendingabók that covers the entire current population and runs back to the origins of the country. Despite being a company (and now a subsidiary of Amgen), deCODE appears to operate just like another research institution, searching for genetic explanations of diseases and genotyping more and more individuals towards that goal, with a startup atmosphere in a well-designed building… A most unusual and enjoyable evening at a conference! Making me wonder if they have visiting positions…

## Móskarðshnjúkar [hiking near Reykjavik]

Posted in Mountains, pictures, Running, Travel with tags , , , , , , , on June 7, 2015 by xi'an

Today, as I had a free day (with 24 hour daylight!) in Reykjavik before the NBBC15 conference started, thanks to the crazy schedules of the low cost sister of Air France, Transavia (!), I went in search of a hike… Which is not very difficult in Iceland! I had originally planned to stop near Geysir as the dirt road beyond Gullfoss is off-limit for rental cars. Especially small 2WD like mine.

As I was driving the first kms of the Þingvellir road, I admired the Esjan range starting with the Esja mountain that we had climbed during our previous visit to Iceland. Especially the “last” peak that glowed with a warm yellow (and apparently no snow at all). More especially, because it had a top reminding me of the Old Man of Storr on its slope. (Not that I could spot it while driving!) And quickly decided this was a great opportunity for a nice hike and a minimum of driving as I was about 20 mn from down-town Reykjavik.

I thus took a dirt road that seemed to get closer to my goal and after 500m came to a farm yard where I parked the car and went hiking, aiming at this peak,  which name is Móskarðshnjúkar. Despite a big cut due to a torrent after the first hill, I managed to keep enough to high ground not to loose any altitude and sticking to the side of the ski station Skálafell (where a few people were still skiing with the noisy help of two snowmobiles), I crossed the brook easily as it was covered by snow and started moving to steeper if manageable slopes. I reached the bottom of the main peak rather quickly and then understood both its colour and the absence of snow.

As maybe visible from some of my pictures (?), the Móskarðshnjúkar peak is covered with gravel in a bright yellow stone that seems to accumulate heat very well. Climbing straight on the loose gravel was then impossible and I had to zigzag mostly up, trying to not lose too much ground to micro-avalanches. As I reached the tor I spotted two hikers above me and when I reached the top I realised there was a path coming from the west, connecting this peak with its neighbours. The normal route seems to come from a gravel road that starts close to Mount Esja, to the west, and as I followed the path down to the saddle between Móskarðshnjúkar and the rest of the range, I saw this path winding down to the valley with further hikers coming up. Before I crossed them, I went up again to the next peak, which was an easy if beautiful ridge walk, with still a fair amount of snow remaining on the north face (heavy enough to bear tracks of snowmobiles!). After following the ridge track for a while, it branched north to reach the main Esja plateau and I left the track to get down a rocky shoulder towards my starting point. However, I had forgotten about the torrent cut between the two ranges and this forced me to take a further detour. And to cross the torrent barefooted, as there was no stone ford on this off-path section. No big drama as the melted snow water was not that cold…

A last sight was provided by the final rocky outcrop, which enjoyed basaltic volcanic columns as on the picture above. A terrific hiking half-day with a sharp sunny weather and not too much wind except at the top. It was very pleasant to walk part of the way on moss and last year grass, with a surprising absence of bogs and mud when compared with Scotland.

## moon under the midnight sun

Posted in Mountains, pictures, Travel, University life with tags , , , , , on June 6, 2015 by xi'an

## 5th Nordic-Baltic Biometric Conference, Reykjavik, Iceland, June 8-10, 2015

Posted in Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , on January 21, 2015 by xi'an

The next Nordic-Baltic Biometric conference will take place in Reykjavik, next June, a few days after the O-Bayes 15 meeting in València. I will attend the conference as the organisers were kind enough to invite me to give a talk, with high hopes to take a few days off to go hiking day and night! The registration is now open, as is the call for abstracts.

## someone to watch over me [Horfðu á mig]

Posted in Books, Travel with tags , , , , , , on September 20, 2014 by xi'an

And yet another roman noir taking place in Iceland! My bedside read over the past two months was “Someone to watch over me” by Yrsa Sigurðardóttir. (It took that long because I was mostly away in July and August, not because the book was boring me to sleep every night!) It is a fairly unusual book in several respects: the setting is an institution for mentally handicapped patients that was set on fire, killing five of the patients as a result, the investigator is an Icelandic lawyer, Þóra Guðmundsdóttir, along with her German unemployed-banker boyfriend, the action takes place at the height [or bottom!] of the Icelandic [and beyond!] economic crisis, when most divorce settlements are about splitting the debts of the household, and when replacing a computer becomes an issue, some of the protagonists, including the main suspects, are mentally ill, and the police and justice are strangely absent from most of the story. The the book tells a lot about the Icelandic society, where a hit-and-run is so unheard of that the police is clueless. Or seems to be. And where people see ghosts. Or think they do, as the author plays (heavily?) on the uncertainty about those ghosts. (At least, there are no elves. Nor trolls.) Definitely more in tune with the “true” Iceland than Available dark. (Well, as far as I can tell!) The mystery itself is a wee bit stretched and the final resolution slightly disappointing, implying some unlikely behaviour from the major characters. In particular, I do not buy the explanation motivating the arson itself. Terrible cover too. And not a great title in English (Watch me or Look at me would have been better) given the many books, movies and songs with the same title. Nonetheless, I liked very much the overall atmosphere of the book, enough to recommend it.