The AstroStat Slog » simulated annealing http://hea-www.harvard.edu/AstroStat/slog Weaving together Astronomy+Statistics+Computer Science+Engineering+Intrumentation, far beyond the growing borders Fri, 09 Sep 2011 17:05:33 +0000 en-US hourly 1 http://wordpress.org/?v=3.4 A History of Markov Chain Monte Carlo http://hea-www.harvard.edu/AstroStat/slog/2008/mcmc-historyo/ http://hea-www.harvard.edu/AstroStat/slog/2008/mcmc-historyo/#comments Wed, 17 Sep 2008 18:11:01 +0000 hlee http://hea-www.harvard.edu/AstroStat/slog/?p=768 I’ve been joking about the astronomers’ fashion in writing Markov chain Monte Carlo (MCMC). Frequently, MCMC was represented by Monte Carlo Markov Chain in astronomical journals. I was curious about the history of this new creation. Overall, I thought it would be worth to learn more about the history of MCMC and this paper was up in arxiv:

[stat.CO:0808.2902] A History of Markov Chain Monte Carlo–Subjective Recollections from Incomplete Data– by C. Robert and G. Casella
Abstract: In this note we attempt to trace the history and development of Markov chain Monte Carlo (MCMC) from its early inception in the late 1940′s through its use today. We see how the earlier stages of the Monte Carlo (MC, not MCMC) research have led to the algorithms currently in use. More importantly, we see how the development of this methodology has not only changed our solutions to problems, but has changed the way we think about problems.

Here is the year list of monumental advances in the MCMC history,

  • 1946: ENIAC
  • late 1940′s: inception along with Monte Carlo methods.
  • 1953: Metropolis algorithm published in Journal of Chemical Physics (Metropolis et al.)
  • 1970: Hastings algorithms in Biometrika (Hastrings)
  • 1974: Gibbs sampler and Hammersley-Clifford theorem paper by Besag and its discussion by Hammersley in JRSSS B
  • 1977: EM algorithm in JRSSS B (Dempster et al)
  • 1983: Simulated Annealing algorithm (Kirkpatrick et al.)
  • 1984: Gibbs sampling in IEEE Trans. Pattern Anal. Mach. Intell. (Geman and Geman, this paper is responsible for the name)
  • 1987: data augmentation in JASA (Tanner and Wong)
  • 1980s: image analysis and spatial statistics enjoyed MCMC algorithms, not popular with others due to the lack of computing power
  • 1990: seminal paper by Gelfand and Smith in JSAS
  • 1991: BUGS was presented at the Valencia meeting
  • 1992: introductory paper by Casella and Georgy
  • 1994: influential MCMC theory paper by Tierney in Ann. Stat.
  • 1995: reversible jump algorithm in Biometrika (Green)
  • mid 1990′s: boom of MCMC due to particle filters, reversible jump and perfect sampling (second-generation of MCMC revolution)

and a nice quote from conclusion.

MCMC changed out emphasis from “closed form” solutions to algorithms, expanded our immpact to solving “real” applied problems, expanded our impact to improving numerical algorithms using statistical ideas, and led us into a world where “exact” now means “simulated”!

If you consider applying MCMC methods in your data analysis, references listed in Robert and Casella serve as a good starting point.

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[ArXiv] A fast Bayesian object detection http://hea-www.harvard.edu/AstroStat/slog/2008/arxiv-a-fast-bayesian-object-detection/ http://hea-www.harvard.edu/AstroStat/slog/2008/arxiv-a-fast-bayesian-object-detection/#comments Wed, 05 Mar 2008 21:46:48 +0000 hlee http://hea-www.harvard.edu/AstroStat/slog/2008/arxiv-a-fast-bayesian-object-detection/ This is a quite long paper that I separated from [Arvix] 4th week, Feb. 2008:
      [astro-ph:0802.3916] P. Carvalho, G. Rocha, & M.P.Hobso
      A fast Bayesian approach to discrete object detection in astronomical datasets – PowellSnakes I
As the title suggests, it describes Bayesian source detection and provides me a chance to learn the foundation of source detection in astronomy.

First, I’d like to point out that my initial concerns from [astro-ph:0707.1611] Probabilistic Cross-Identification of Astronomical Sources are explained in sections 2, 3 and 6 about parameter space, its dimensionality, and priors in Bayesian model selection.

Second, I’d rather concisely list the contents of the paper as follows: (1) priors, various types but rooms were left for further investigations in future; (2) templates (such as point spread function, I guess), crucial for defining sources, and gaussian random field for noise; (3) optimization strategies for fast computation (source detection implies finding maxima and integration for evidence); (4) comparison with other works; (5) upper bound, tuning the threshold for acceptance/rejection to minimize the symmetric loss; (6) challenges of dealing likelihoods in Fourier space from incorporating colored noise (opposite to white noise); (7) decision theory from computing false negatives (undetected objects) and false positives (spurious objects). Many issues in computing Bayesian evidence, priors, tunning parameter relevant posteriors, and the peaks of maximum likelihoods; and approximating templates and backgrounds are carefully presented. The conclusion summarizes their PowellSnakes algorithm pictorially.

Thirdly, although my understanding of object detection and linking it to Bayesian techniques is very superficial, my reading this paper tells me that they propose some clever ways of searching full 4 dimensional space via Powell minimization (It seems to be related with profile likelihoods for a fast computation but it was not explicitly mentioned) and the detail could direct statisticians’ attentions for the improvement of computing efficiency and acceleration.

Fourth, I’d like to talk about my new knowledge that I acquired from this paper about errors in astronomy. Statisticians usually surprise at astronomical catalogs that in general come with errors next to single measurements. These errors are not measurement errors (errors calculated from repeated observations) but obtained from Fisher information owing to Cramer-Rao Lower Bound. The template likelihood function leads this uncertainty measure on each observation.

Lastly, in astronomy, there are many empirical rules, effects, and laws that bear uncommon names. Generally these are sophisticated rules of thumb or approximations of some phenomenon (for instance, Hubble’s law, though it’s well known) but they have been the driving away factors when statisticians reading astronomy papers. On the other hand, despite overwhelming names, when it gets to the point, the objective of mentioning such names is very statistical like regression (fitting), estimating parameters and their uncertainty, goodness-of-fit, truncated data, fast optimization algorithms, machine learning, etc. This paper mentions Sunyaev-Zel’dovich effect, which name scared me but I’d like to emphasize that this kind nomenclature may hinder from understanding details but could not block any collaborations.

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