In theory, the probability of correct responses ranges between 0.5  
and 1 here,
but in practice it is frequent to find cases where the observed  
proportion of correct
responses is a little less.  The number of trials is limited, after  
all.  The inverse
of this link function generates a Nan when this occurs.  Is that a  
problem?
and if so, how can it be dealt with here?

Thank you.

I have used the gnlr function in Lindsey's gnlm package for this  
problem in
the past, but glm would be simpler, it seems to me.

@Article{pmid16817511,
    Author="Yssaad-Fesselier, Rosa and Knoblauch, Kenneth",
    Title="{{M}odeling psychometric functions in {R}}",
    Journal="Behav Res Methods",
    Year="2006",
    Volume="38",
    Number="1",
    Pages="28--41",
    Month="Feb"
}



> On Sat, 16 Dec 2006, baud-bovy.gabriel at hsr.it wrote:
>
> > I have would like to use logistic regression to analyze the
> > percentage of correct responses in a 2 alternative forced
> > choice task. The question is whether one needs to take into
> > account the fact expected probabilities for the percentage of
> > correct responses ranges between 0.5 and 1 in this case.
>
> Yes.
>
> > Second, how can one implement a link function of the
> > type f(x) = (1+exp(x)/(1+exp(x)))/2 in R?
>
> Looking at make.link() should give you enough to go on.
>
> > Third, can it be also done with gee and/or glmm?
>
> For gee, you need to change the C-level internals. (I've done this  
> in the
> far past for S-PLUS but not for R.)  It would be easier to use yags  
> (but I
> think you still need to dive into the internals).
>
> What 'glmm' did you have in mind?  Looks like e.g. glmmML and  
> glmmPQL will
> work with the new link.
>
> [...]
>
> Someone may have been here already: e.g.
> http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1434755
>
> -- 
> Brian D. Ripley,                  ripley at stats.ox.ac.uk
> Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
> University of Oxford,             Tel:  +44 1865 272861 (self)
> 1 South Parks Road,                     +44 1865 272866 (PA)
> Oxford OX1 3TG, UK                Fax:  +44 1865 272595

--
Ken Knoblauch
Inserm U371
Institut Cellule Souche et Cerveau
Département Neurosciences Intégratives
18 avenue du Doyen Lépine
69500 Bron
France
tel: +33 (0)4 72 91 34 77
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