Archive for the ‘Fitting’ Category.

Apr 30th, 2010| 12:12 pm | Posted by vlk

Sherpa is a fitting environment in which Chandra data (and really, X-ray data from any observatory) can be analyzed. It has just undergone a major update and now runs on python. Or allows python to run. Something like that. It is a very powerful tool, but I can never remember how to use it, and I have an amazing knack for not finding what I need in the documentation. So here is a little cheat sheet (which I will keep updating ~~as and when~~ if I learn more): Continue reading ‘Everybody needs crampons’ »

Tags:

Chandra,

cheat sheet,

ciao,

how to,

Python,

Sherpa,

Sherpa4 Category:

Algorithms,

Astro,

Fitting,

Jargon,

Languages |

2 Comments
Dec 20th, 2009| 07:27 pm | Posted by hlee

Tags:

assumptions,

bulletin,

IMS,

maximum likelihood,

MLE,

T. Speed Category:

arXiv,

Cross-Cultural,

Fitting,

Frequentist,

Methods,

Stat |

1 Comment
Nov 21st, 2009| 05:06 am | Posted by hlee

by Emanuel Parzen in * Statistical Science* 2004, Vol 19(4), pp.652-662 JSTOR

I teach that statistics (done the quantile way) can be simultaneously frequentist and Bayesian, confidence intervals and credible intervals, parametric and nonparametric, continuous and discrete data. My first step in data modeling is identification of parametric models; if they do not fit, we provide nonparametric models for fitting and simulating the data. The practice of statistics, and the modeling (mining) of data, can be elegant and provide intellectual and sensual pleasure. Fitting distributions to data is an important industry in which statisticians are not yet vendors. We believe that unifications of statistical methods can enable us to advertise, “What is your question? Statisticians have answers!”

I couldn’t help liking this paragraph because of its bitter-sweetness. I hope you appreciate it as much as I did.

Oct 23rd, 2009| 12:26 pm | Posted by hlee

I watched a movie in which one of the characters said, “*country A has nukes with 80% chance*” (perhaps, not 80% but it was a high percentage). One of the statements in that episode is that *people will not eat lettuce only if the 1% chance of e coli is reported, even lower. Therefore, with such a high percentage of having nukes, it is right to send troops to A.* This episode immediately brought me a thought about astronomers’ null hypothesis probability and their ways of concluding chi-square goodness of fit tests, likelihood ratio tests, or F-tests.

First of all, I’d like to ask how you would like to estimate the chance of having nukes in a country? What this 80% implies here? But, before getting to the question, I’d like to discuss computing the chance of e coli infection, first. Continue reading ‘The chance that A has nukes is p%’ »

Tags:

chances,

chi-square statistic,

composite likelihood,

delta chi-square,

F-test,

fiducial likelihood,

likelihood,

LRT,

p-value,

posterior,

prior Category:

Bayesian,

Cross-Cultural,

Fitting,

Frequentist,

Misc,

Quotes,

Uncertainty |

Comment
Oct 15th, 2009| 06:46 pm | Posted by hlee

Astronomers rely on scatter plots to illustrate correlations and trends among many pairs of variables more than any scientists^{[1]}. Pages of scatter plots with regression lines are often found from which the slope of regression line and errors bars are indicators of degrees of correlation. Sometimes, too many of such scatter plots makes me think that, overall, resources for drawing nice scatter plots and papers where those plots are printed are wasted. Why not just compute correlation coefficients and its error and publicize the processed data for computing correlations, not the full data, so that others can verify the computation results for the sake of validation? A couple of scatter plots are fine but when I see dozens of them, I lost my focus. This is another cultural difference. Continue reading ‘Scatter plots and ANCOVA’ »

Tags:

ANCOVA,

ANOVA,

approximation,

correlation,

Gaussianity,

graphics,

MADS,

modeling,

nonparametric,

parallel coordinates,

PCA,

quality,

quantity,

regression,

scatter plots Category:

arXiv,

Cross-Cultural,

Fitting,

Jargon,

Methods,

Stat,

Uncertainty |

Comment
Oct 13th, 2009| 03:15 pm | Posted by hlee

Although a bit of time has elapsed since my post space weather, saying that **logistic regression** is used for prediction, it looks like still true that **logistic regression** is rarely used in astronomy. Otherwise, it could have been used for the similar purpose not under the same statistical jargon but under the Bayesian modeling procedures. Continue reading ‘[MADS] logistic regression’ »

Sep 11th, 2009| 03:40 pm | Posted by hlee

A number of practical Bayesian data analysis books are available these days. Here, I’d like to introduce two that were relatively recently published. I like the fact that they are rather technical than theoretical. They have practical examples close to be related with astronomical data. They have R codes so that one can try algorithms on the fly instead of jamming probability theories. Continue reading ‘[Books] Bayesian Computations’ »

Tags:

book,

BUGS,

CMB,

examples,

HMM,

identifiability,

image processing,

LLN,

mixture,

MRF,

R Category:

Bayesian,

Fitting,

Languages,

MC,

MCMC,

Methods,

Stat |

1 Comment
Jun 12th, 2009| 03:47 pm | Posted by hlee

**A Fast Thresholded Landweber Algorithm for Wavelet-Regularized Multidimensional Deconvolution**

Vonesch and Unser (2008)

IEEE Trans. Image Proc. vol. 17(4), pp. 539-549

Quoting the authors, I also like to say that __the recovery of the original image from the observed is an __**ill-posed** problem. They traced the efforts of wavelet regularization in deconvolution back to a few relatively recent publications by astronomers. Therefore, I guess the topic and algorithm of this paper could drag some attentions from astronomers. Continue reading ‘Wavelet-regularized image deconvolution’ »

Tags:

bound optimization,

deconvolution,

image processing,

impulse response,

MM algorithm,

PSF,

regularization,

restoration,

thresholding,

wavelet Category:

Algorithms,

arXiv,

Data Processing,

Fitting,

Imaging,

Jargon,

Methods,

Quotes,

Stat |

Comment
Jun 2nd, 2009| 03:03 am | Posted by hlee

Even though I traced the astronomers’ casual usage of the **null hypothesis probability** in a fashion of reporting outputs from data analysis packages of their choice, there were still some curious cases of the **null hypothesis probability** that I couldn’t solve. They are quite mysterious to me. Sometimes too much creativity harms the original intention. Here are some examples. Continue reading ‘Curious Cases of the Null Hypothesis Probability’ »

Tags:

cases,

chi-sq,

curious,

degree of freedom,

dof,

F-test,

goodness-of-fit test,

Model Selection,

null hypothesis probability,

p-value,

reduced chi-sq Category:

arXiv,

Astro,

Cross-Cultural,

Fitting,

Methods,

Uncertainty |

3 Comments
Mar 17th, 2009| 03:37 pm | Posted by hlee

I couldn’t believe my eyes when I saw 4754 degrees of freedom (d.f.) and chi-square test statistic 4859. I’ve often enough seen large degrees of freedom from journals in astronomy, several hundreds to a few thousands, but I never felt comfortable at these big numbers. Then with a great shock 4754 d.f. appeared. I must find out why I feel so bothered at these huge degrees of freedom. Continue reading ‘4754 d.f.’ »

Tags:

Binning,

chi-square,

chi-square minimization,

chi-square optimization,

chi-square statistic,

class,

degrees-of-freedom,

equiprobable,

goodness-of-fit test,

kernel density estimation Category:

Bad AstroStat,

Fitting,

High-Energy,

Methods,

Spectral,

X-ray |

2 Comments
Feb 12th, 2009| 02:03 pm | Posted by hlee

Student’s t-distribution is somewhat underrepresented in the astronomical community. Having an article with nice stories, it looks to me the best way to introduce the t distribution. This article describing historic anecdotes about monumental statistical developments occurred about 100 years ago.

Guinness, Gosset, Fisher, and Small Samples by Joan Fisher Box

Source: Statist. Sci. Volume 2, Number 1 (1987), 45-52.

No time for reading the whole article? I hope you have a few minutes to read following quotes, which are quite enchanting to me. Continue reading ‘Guinness, Gosset, Fisher, and Small Samples’ »

Tags:

distribution,

error,

Gosset,

guinness,

history,

sampling distribution,

small sample,

student t Category:

arXiv,

Fitting,

Frequentist,

Quotes,

Stat |

Comment
Jan 15th, 2009| 06:01 pm | Posted by hlee

I wonder what Fisher, Neyman, and Pearson would say if they see “Technique” after “Likelihood Ratio” instead of “Test.” A presenter’s saying “Likelihood Ratio Technique” for source identification, I couldn’t resist checking it out not to offend founding fathers of the likelihood principle in statistics since “Technique” sounded derogatory to be attached with “Likelihood” to my ears. I thank, above all, the speaker who kindly gave me the reference about this likelihood ratio technique. Continue reading ‘Likelihood Ratio Technique’ »

Tags:

Fisher,

likelihood principle,

likelihood ratio technique,

likelihood ratio test,

Neyman,

Pearson Category:

Algorithms,

arXiv,

Astro,

Bayesian,

Cross-Cultural,

Data Processing,

Fitting,

Frequentist,

Jargon,

Methods,

Objects,

Stat,

Uncertainty |

Comment
Nov 17th, 2008| 01:39 pm | Posted by hlee

The full description is given http://cxc.harvard.edu/ciao3.4/ahelp/bayes.html about “bayes” under sherpa/ciao^{[1]}. Some sentences kept bothering me and here’s my account for the reason given outside of quotes. Continue reading ‘It bothers me.’ »

Tags:

bayes,

ciao,

ML,

Sherpa Category:

Algorithms,

Astro,

Cross-Cultural,

Data Processing,

Fitting,

High-Energy,

Jargon,

Languages,

Methods,

Spectral,

Uncertainty,

X-ray |

4 Comments
Nov 6th, 2008| 06:00 am | Posted by vlk

**[9/30/2008]**

The CfA is celebrating the 100th anniversary of the discovery of the Cepheid period-luminosity relation on Nov 6, 2008. See http://www.cfa.harvard.edu/events/2008/leavitt/ for details.

**[Update 10/03]** For a nice introduction to the story of Henrietta Swan Leavitt, listen to this Perimeter Institute talk by George Johnson: http://pirsa.org/06050003/

**[Update 11/06]** The full program is now available. The symposium begins at Noon today.

Tags:

2008,

cepheid,

CfA,

Henrietta Leavitt,

Henrietta Swan Leavitt,

history,

November,

period-luminosity,

symposium Category:

Astro,

Fitting,

News,

Stars,

Timing |

1 Comment
Oct 23rd, 2008| 12:13 pm | Posted by hlee

I’ve talked about IMSL on my pyIMSL post, which is a commercial scientific library. There is a GNU version of IMSL, **GSL.** Finding GSL is the courtesy of Jiangang, who was the author of the poster that I most liked from the 212th AAS, (see *My first AAS. V. measurement error and EM* and his comment.) Continue reading ‘GSL – GNU Scientific Library’ »