Yes, indeed, the ltm package has the function tpm() that can fit
Birnbaum's three parameter model that allows for a guessing parameter
(i.e., when the ability levels \theta -> -Inf, the probability for
correct response is allowed to be nonzero) but it does not have a
function to incorporate an upper asymptote. I am not aware of any
other R package that can fit this model.
According to my experience, the models with asymptotes are not
particularly numerically stable, especially when the guessing
parameters are practically zero (i.e., the maximum is on the boundary
of the parameter space). In such cases, the 'constraint' of tpm() can
be used to set the guessing parameters for some items to zero and
investigate data set at hand.
I hope it helps.
Best,
Dimitris
Quoting "Doran, Harold" <[EMAIL PROTECTED]>:
I am pretty certain a function for this model does not exist. Jan
Deleeuw or Dimitris Rizopolous may suggest otherwise. There is a package
for a model that would allow for the lower asymptote of the function to
be > 0; it does not however, allow the upper asymptote to vary from 1
(well, it tends to 1) and this is in the ltm package.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Mario Maiworm
Sent: Thursday, August 21, 2008 7:05 AM
To: r-help@r-project.org
Subject: [R] psychometric functions
Hi,
I want to fit some psychophysical data with cumulative
gaussians. There is quite a convenient toolbox for matlab
called 'psignifit' (formerly known as 'psychofit'). It allows
the lower bound of the sigmoid to vary slightly from zero,
aswell as the upper bound to vary from one. with these two
free parameters, the fitted function is less sensitive to
noisy data and outliers.
Apart from advertising this toolbox I want to ask for
possibilities in R to fit psychometric functions, as I would
rather use R than matlab. Is there a comparable package
specific for psychophysics in R? otherwise: which function
would be a good choice? Can I have functionality equal to
'psignifit' with glm() from MASS. I would be grateful for
some suggestions of people who have experience in
sigmoid-fitting in R.
Best,
Mario.
__________________________________________________________________
Mario Maiworm
Biological Psychology and Neuropsychology University of
Hamburg Von-Melle-Park 11
D-20146 Hamburg
Phone: +49 40 42838 8265
Fax: +49 40 42838 6591
http://bpn.uni-hamburg.de/Maiworm_e.html
http://cinacs.org
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--
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven
Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
http://perswww.kuleuven.be/dimitris_rizopoulos/
Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm
______________________________________________
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
