[R] psychometric functions

[Dimitris Rizopoulos]

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" <HDoran at air.org>:

> 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: r-help-bounces at r-project.org
>> [mailto:r-help-bounces at r-project.org] On Behalf Of Mario Maiworm
>> Sent: Thursday, August 21, 2008 7:05 AM
>> To: r-help at 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
>>
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> PLEASE do read the posting guide
>> http://www.R-project.org/posting-guide.html
>> and provide commented, minimal, self-contained, reproducible code.
>>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
>



-- 
Dimitris Rizopoulos
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
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