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I'm fitting a psychometric function to a range of data. The majority of this data lends itself to a sigmoidal fit (i.e. participants can do the task), but some individuals are absolutely unable to do the task. I'm planning to compare the slopes obtained from different conditions, but I've hit a wall with the unable-to-do-the-task data.

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Fitting a function to this data, the slope should be nearly flat, right? However, the data is really noisy and some weird fitting is occurring - I end up getting erroneously high slopes. I'm using pypsignifit, the parameters I'm using can be seen below. Any idea how to stop this happening?

num_of_block  = 7
num_of_trials = 20

stimulus_intensities=[3, 7, 13, 20, 27, 32, 39] # stimulus levels
percent_correct=[.38, .75, .6, .43, .7, .65, .43]     # percent correct sessions 1-3
num_observations     = [num_of_trials] * num_of_block      # observations per block
data= np.c_[stimulus_intensities, percent_correct, num_observations]
nafc = 1
constraints = ('unconstrained', 'unconstrained', 'unconstrained', 'Beta(2,20)' )
boot = psi.BootstrapInference ( data, core='ab', sigmoid='gauss', priors=constraints, nafc=nafc )
print 'pse', boot.getThres(0.5)
print 'slope', boot.getSlope()
print 'jnd', (boot.getThres(0.75)-boot.getThres(0.25))
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The data look really like the participant performed on chance level. Actually, I would not try to fit them at all, because the fits cannot get better than your example. –  H.Muster Jun 3 '12 at 17:09
That's exactly what happened. It may be counterintuitive to try to fit the data, but I really want to be able to compare the slope of these participants with ones who performed better using something like a t-test. Hence the 'need' to fit. –  luser Jun 3 '12 at 18:09
Then I would try different psychometric functions (e.g, logistic, Weibull) until I find one that is fitted to the data as a straight line with a slope of almost zero. –  H.Muster Jun 3 '12 at 19:30
Makes sense, but the main issue I have is I'd have to arbitrarily choose when to use this alternative fit. The same participant appeared to perform at chance on another condition, but the cumulative gaussian fitted well and showed a not-immediately-apparent slope. –  luser Jun 3 '12 at 20:08
I would not suggest to use different functions for individual fits, but use one functions for all, i.e., choose the function that gives the best overall solutions. Concerning the bad cases: did you try to seed the estimation routine with starting values nearer to 0.5 for the guessing rate and the lapsing rate? –  H.Muster Jun 4 '12 at 5:56
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