[R] Request for help (reference details)

[Huntsinger, Reid]

I referred in my reply to a paper by Diaconis and Sturmfels. The exact
reference is:

Diaconis and Sturmfels, Algebraic algorithms for sampling from conditional
distributions, Ann. Stat 26 (1998) 363-397. 

They cite the following:

Besag and Clifford, Generalized Monte Carlo significance tests, Biometrika
76 (1989) 633-42.

which actually contains your problem (section 3, Testing the Rasch model)
and gives a very simple Markov chain for sampling from the uniform
distribution on these matrices. If you need other than the uniform
distribution, see the modifications Diaconis and Sturmfels make (the
"Metropolis" step).

Reid Huntsinger

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Huntsinger, Reid
Sent: Thursday, January 27, 2005 10:50 AM
To: 'Michela Marignani'; r-help at stat.math.ethz.ch
Subject: RE: [R] Request for help


Persi Diaconis and Bernd Sturmfels have an article on generating random
contingency tables uniformly distributed subject to having fixed marginals
for the same purpose (null distribution of conditional test) and they used
Markov Chain Monte Carlo to sample. That could perhaps be adapted here. The
article is in Annals of Statistics from several years ago, and if you google
for "algebraic statistics" you'll probably find several recent expositions
of the ideas, possibly even code.

Reid Huntsinger

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Michela Marignani
Sent: Thursday, January 27, 2005 3:52 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Request for help


My name is Michela Marignani and I'm an ecologist trying to solve a problem 
linked to knight' s tour algorithm.
I need a program to create random matrices with presence/absence (i.e. 1,0 
values), with defined colums and rows sums, to create null models for 
statistical comparison of species distribution phenomena.
I've seen on the web many solutions of the problem, but none provides the 
freedom to easily change row+colums constraint and none of them produce 
matrices  with 1 and 0. Also, I've tryied to use R, but it is too 
complicated for a not-statistician as I am....can you help me?

Thank you for your attention,
so long

Michela Marignani
University of Siena
Environmental Science Dept.
Siena, Italy
michela.marignani at uniroma1.it

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