Thank you. This was very helpfull.
Thierry
ir. Thierry Onkelinx
Instituut voor natuur- en bosonderzoek / Research Institute for Nature
and Forest
Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
methodology and quality assurance
Gaverstraat 4
9500 Geraardsbergen
Belgium
tel. + 32 54/436 185
thierry.onkel...@inbo.be
www.inbo.be
To call in the statistician after the experiment is done may be no more
than asking him to perform a post-mortem examination: he may be able to
say what the experiment died of.
~ Sir Ronald Aylmer Fisher
The plural of anecdote is not data.
~ Roger Brinner
The combination of some data and an aching desire for an answer does not
ensure that a reasonable answer can be extracted from a given body of
data.
~ John Tukey
-Oorspronkelijk bericht-
Van: Charles C. Berry [mailto:cbe...@tajo.ucsd.edu]
Verzonden: vrijdag 17 april 2009 21:31
Aan: ONKELINX, Thierry
CC: r-help@r-project.org
Onderwerp: Re: [R] Generate bivariate binomial data
On Fri, 17 Apr 2009, ONKELINX, Thierry wrote:
> Dear all,
>
> Could someone point me to a function or algorithm to generate random
> bivariate binomial data?
>
> Some details about what I'm trying to do. I have a dataset of trees
who
> were categorised as not damaged or damaged. Each tree is measured
twice
> (once in two consecutive years). The trees can recover from the damage
> but the data is clearly correlated. As a (un)damaged tree is more
likely
> to stay (un)damaged. A GEE-model indicates a correlation of about 0.6
> between the measures on the same tree.
The commonly used sampling models for such count data (see Bishop,
Fienberg, and Holland, Discrete Multivariate Analysis, 1975.) involve
four
parameters. There are various parameterizations.
In your case, the total sample size (N), the proportion of
undamaged trees at the first (pr.undam.1), and the proportions at the
second time conditional on the first (p.undam.2.undam.1,
p.undam.2.dam.1)
seems like reasonable way to parameterize the problem to do your
simulation.
If you have the marginal counts and the correlation, you can transform
them to the above parameterization by hacking through the algebra to
find
the expected 2 by 2 table of counts as a function of the latter
parameters.
Then
y <- rbinom( N, 1, pr.undam.1 )
x <- rbinom( N, 1, ifelse(y==1, pr.undam.2.undam.1, pr.undam.2.dam.1 )
table(x,y)
should get you started
HTH,
Chuck
> I need to simulate similar dataset to do some power calculations.
>
> Best regards,
>
> Thierry
>
>
>
> ir. Thierry Onkelinx
> Instituut voor natuur- en bosonderzoek / Research Institute for Nature
> and Forest
> Cel biometrie, methodologie en kwaliteitszorg / Section biometrics,
> methodology and quality assurance
> Gaverstraat 4
> 9500 Geraardsbergen
> Belgium
> tel. + 32 54/436 185
> thierry.onkel...@inbo.be
> www.inbo.be
>
> To call in the statistician after the experiment is done may be no
more
> than asking him to perform a post-mortem examination: he may be able
to
> say what the experiment died of.
> ~ Sir Ronald Aylmer Fisher
>
> The plural of anecdote is not data.
> ~ Roger Brinner
>
> The combination of some data and an aching desire for an answer does
not
> ensure that a reasonable answer can be extracted from a given body of
> data.
> ~ John Tukey
>
> Dit bericht en eventuele bijlagen geven enkel de visie van de
schrijver weer
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bevestigd is
> door een geldig ondertekend document. The views expressed in this
message
> and any annex are purely those of the writer and may not be regarded
as stating
> an official position of INBO, as long as the message is not confirmed
by a duly
> signed document.
>
> __
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>
Charles C. Berry(858) 534-2098
Dept of Family/Preventive
Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego
92093-0901
Dit bericht en eventuele bijlagen geven enkel de visie van de schrijver weer
en binden het INBO onder geen enkel beding, zolang dit bericht niet bevestigd is
door een geldig ondertekend document. The views expressed in this message
and any annex are purely those of the writer and may not be regarded as stating
an official position of INBO, as long as the mess
