How to Fix Parameters

Sometimes one may need to, or want to explore the effect of, fixing one or more parameters when estimating the psychometric function. In psignifit 4 you can do so by setting the options.fixedPars field. This is a vector with 5 entries for the five parameters in usual order (threshold, width, lapse, guess and eta). Enter a NaN for an unfixed parameter and a the number you wish to enforce for parameters you want to fix.

For example

options.fixedPars = NaN(5,1);
options.fixedPars(3) = .01;

fixes the lapse rate at 1% (= 0.01).

And the following fixes all parameters except the threshold and the width

options.fixedPars = [NaN;NaN;.01,.5,0];

Note that this is a valid input to psignifit 4 but not generally recommended as an analysis strategy!