# Confused about using Quest

**URL:** <https://psychtoolbox.discourse.group/t/confused-about-using-quest/1502>\
**Category:** Uncategorized\
**Created:** [January 15, 2008, 11:30pm UTC](https://psychtoolbox.discourse.group/t/confused-about-using-quest/1502 "2008-01-15T23:30:56Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![jackm\_ustc](https://avatars.discourse-cdn.com/v4/letter/j/c77e96/32.png) [@jackm\_ustc](https://psychtoolbox.discourse.group/u/jackm_ustc)\
**Post date:** [January 15, 2008, 11:30pm UTC](https://psychtoolbox.discourse.group/t/confused-about-using-quest/1502/1 "2008-01-15T23:30:56Z")

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Hi PTB experts,  
  
You may have some experience on using Quest to measure thresholds.   
Being a subject, you would inevitably make some silly responses. For   
example, when the stimulus intensity is pretty high you actually know   
the correct response should be a press on one button. But sometimes   
you press others just because of absent-minded, impulsion or   
carelessness. You might try your best not to make mistakes, but I   
think nobody can entirely avoid this.  
  
I thereby try testing how much such kind of mistakes would have a bad   
effect on the final estimated threshold and SD. In the initial stage   
of a quest, the stimuli intensities are often very high or very low.   
After some responses, say 10 or more, the test intensity will   
fluctuate around a value within a small range. If the first several   
responses involve an aforementioned silly response, will the accuracy   
and variation of the estimation become bad? Therefore, I made a code   
attached below to seek the answer. It seems the SD of the estimated   
threshold increase prominently when there is only 1 silly response in   
the intial stage of a quest. When there are 2 silly responses, not   
only the SD but also the estimated threshold become very unreliable.  
  
Since there are so many papers (more than 430) citing Prof.Pelli's   
first introduction on quest (Pelli, 1983), it's very difficult to   
find a paper that can deal with the silly responses happenning in the   
intial stage of a quest? Does anybody know the solution?  
  
%This is my code. If you have interst, you can try running it.  
%  
  
thresh1 = []; sd1 = []; thresh2 = []; sd2 = [];  
tActual = -2; tGuess = log10(.015); tGuessSd = 2;  
pThreshold=0.82;  
beta=3.5;delta=0.01;gamma=0.5;trialsDesired=40;wrongRight=  
{'wrong','right'};  
for i = 1:50  
 q=QuestCreate(tGuess,tGuessSd,pThreshold,beta,delta,gamma);  
 q.normalizePdf=1;  
 for k=1:trialsDesired  
 tTest=QuestQuantile(q);  
 response=QuestSimulate(q,tTest,tActual);  
 q=QuestUpdate(q,tTest,response);  
 end  
 t=QuestMean(q);  
 sd=QuestSd(q);  
 thresh1 = [thresh1 t];  
 sd1 = [sd1 sd];  
end  
  
kkk1 = Randi(10)+3; kkk2 = Randi(10)+3;  
for i = 1:50  
 q=QuestCreate(tGuess,tGuessSd,pThreshold,beta,delta,gamma);  
 q.normalizePdf=1;  
 for k=1:trialsDesired  
 tTest=QuestQuantile(q);  
 response=QuestSimulate(q,tTest,tActual);  
 %if k == kkk1  
 if k == kkk1 | k == kkk2  
 response = abs(response-1);  
 end  
 q=QuestUpdate(q,tTest,response);  
 end  
 t=QuestMean(q);  
 sd=QuestSd(q);  
 thresh2 = [thresh2 t];  
 sd2 = [sd2 sd];  
end  
[mean(thresh1) std(thresh1) mean(sd1) std(sd1);mean(thresh2) std  
(thresh2) mean(sd2) std(sd2)]  
[min(thresh1) max(thresh1) min(sd1) max(sd1);min(thresh2) max  
(thresh2) min(sd2) max(sd2)]