[R] Psychometric curves, two altnerative force choice, glm, and budbworms

[Peter Dalgaard BSA]

"Dieter Menne" <dieter.menne at menne-biomed.de> writes:

> Dear R-Listers,
> 
> to measure the psychometric curve of pitch discrimination, one sequentially
> presents two tones of slightly different pitch to an observer (animal will
> do), and asks "which is higher". The pschometric curve is the fraction of
> correct responses plotted against the pitch difference. It passes through
> 50% (pure guessing) at zero and normally approaches 100% at large
> difference.
> To compare two psychometric curves, the conventional way is to fit two
> logistic curves and compare the 75% correct "threshold" values (whatever
> threshold means).
> 
> I want to handle the case similar to the budworm example in MASS
> (glm(SF~sex*ldose, family=binomial)). My basic idea is that the 2AFC forced
> choice psychometric curve, normally only defined for positive stimuls
> differences, could conceptually be continued to negative values by mirroring
> the values at (0,0.5) to get the whole binomial/logistic curve. As far as I
> can see, the result would be correct if we divide the counts for a stimulus
> difference equally between positive and negative branch, with correct/wrong
> inverted for the negatives. The solution is not clean if we have uneven
> counts.
> 
> Is this a valid approach? Alternatively, could I force a
> glm/binomial/logistic fit to go through 0/0.5?

Sure, just remove the intercept and change the response definition
from "correct" to "chooses A" (i.e. "correct" if delta is positive,
"wrong" if it is negative). glm(chooseA~delta-1, binomial)

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)             FAX: (+45) 35327907
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