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QuestHandler doesn't seek trough enough space

I am trying to modify Tutorial 2 to use the QuestHandler to staircase the distance between two objects, moving them further away from the centre with correct guesses and vice versa.

I think the relevant piece of the code looks like this

staircase = data.QuestHandler(10, startValSd=9,
    pThreshold=0.63, gamma=0.5,
    nTrials=50, minVal=0, maxVal=25)

I have set the start to be 10 and the min/max to be 0/25, but the staircase quickly reduces the step-distance even if every single answer is correct or wrong. In the present example. It will not walk above/below 7.5/12.5

My intuition is that it has something to do with the startValSd, but I have not achieved anything useful thus far by manipulating it.

I hope some of you can help me on.

Here is how to reproduce the ‘error’. This function assumes that a participant always hit over 80 trials. As you can see it starts at 10 and never goes below 7.5 even though the minimum value should be 0.

from psychopy import core, visual, gui, data, event

# create the staircase handler
staircase = data.QuestHandler(10, startValSd=9,
    pThreshold=0.63, gamma=0.5,
    nTrials=80, minVal=0, maxVal=25)

# walk trough an all correct staircase    
for thisIncrement in staircase:
    value1 = thisIncrement

I found a solution by adding

range = XX