Archive for determinism

MCMskv #1 [room with a view]

Posted in Mountains, pictures, Statistics, Travel, University life with tags , , , , , , , , , , , on January 6, 2016 by xi'an

That’s it!, MCMskv has now started! We hold our round-table Monday night, which ended with most of my interventions revolving about the importance of models. And of the fact that models are always approximate (and wrong), hence that uncertainty and uncertainty ascertainment is paramount. Even more with large datasets and roundtablehigh-dimensional models. Apologies to the audience if I sounded like running on a very short loop. (And maybe also for the round-table to keep them from their dinner!)  Still, I got some items for reflection out of this discussion, including the notion that big data is usually and inappropriately associated with an impression of completeness that is almost deterministic in a Laplacian sense. Namely that the available data for, say, all Facebook users, seems to allow us (or The Machine) to play Laplace’s Demon. And thus forgoes the need for uncertainty and uncertainty ascertainment. Which obviously clashes with the issues of poor data, inappropriate models, and time or space stationarity of the available information.

Two more computing-related notions that came out the discussion [for me] are asynchronicity (in the sense explored by Terenin et al. a few months ago) and subsampling, The later seems to mean many things, judging from the discussion from the panel and the audience. For me, it corresponded to the ability (or inability) to handle only part of the available data to simulate the posterior associated with this available data.

The first talk on Tuesday morning was the plenary talk by Michael Jordan about his incorporation of complexity constraints on the convergence of an MCMC variable selection algorithm. (I though I had commented this paper in the past on the ‘Og but apparently I did not!) This was quite interesting, with ultra-fast convergence of the sampler. The talk was alas made harder to follow because of a cameraman standing in front of most of the audience for the entire time, as in the above picture. (I also noticed the interesting randomness of the light panels, who all display different patterns of dots, maybe random enough to satisfy a randomness test!) Another if irrelevant annoying fact was that I discovered upon arrival that my airbnb rental was located 8 kilometres away from the conference location, in a completely different town! Thankfully, we had rented a car [for 5] which saved the day (and even more the night!).

Determinism versus randomness in Biology

Posted in Statistics with tags , on October 10, 2009 by xi'an

As mentioned earlier, I bought the current issue of La Recherche for its catchy special report on the impact of randomness on gene expressions and other biological events. The absurd graph was actually a good  warning about the quality of the report: there is nothing there but a succession of evidences, cut by empty generalities on the nature of randomness like “determinism is only a special case of randomness”, “random does not mean unpredictable” and “randomness does not imply an absence of reproducibility”… The type of determinism invoked in the articles seems to be related to the basic Cartesian type of determinism where a given cause always induces the same effect, which would mean that the same regulatory proteins induce the same expression on all genes at all times. Given that these proteins are in small quantities, thus are not available for all genes, it seems to me quite logical that some genes are expressed and others are not! The argument is therefore not even anti-deterministic but gives a finer reason to the source of the expression mechanism. Randomness at the molecular level thus appears as an evidence and the report does not shed any light on why this “theory” has had so much trouble to penetrate the deterministic foundations of Biology. This is a wasted opportunity as I think there are fairly interesting issues in this domain, in particular in trying to explain different reactions of a cell under identical energy  input and environmental conditions. As usual, I find the journal has a terrible tendency to oversimplifiy topics to the point of making them uninteresting.

Ps-A gold medal to the sentence that “throwing a [fair] coin 100,000 times will result in about 50,000 heads”, missing the fact that the standard deviation is then of 500, i.e. that the outcome is 50,000±1000… And a silver one for missing the religious meaning of probabilism.

Bad graph of the day

Posted in Books, Statistics with tags , , , , on October 7, 2009 by xi'an

The French general science monthly La Recherche published in its October issue a graph about the difference between randomness and determinism which, besides being useless and meaningless (with the caption “determinism, a case of probabilism“, as if probability was a kind of disease!), shares the same feature as a density plot I saw in The Economist when looking at an abridged Black Swan, namely that it shows two densities with the same modal value and with two strongly different areas. (I’ll comment later on the whole issue of separating determinism from randomness.)