Cut-off Value in Survery

I am also conducting research on the effect of High-Definition transcranial direct stimulation (HD-tDCS) on implicit motor sequence learning, via SRTT (serial reaction time task). 20 participants receive 30min stimulation, 20 receive 15min, and 20 are the control group (sham) and receive no stimulation.
In order to exclude the data of participants whose reaction times may have been affected by the stimulation (discomfort, itching, distraction, slight burning, etc.), participants must fill out an "unpleasantness" survey, consisting of several questions to their perceived unpleasantness.
My question is: How can I empirically/ statistically choose a cut-off value, in order to determine from which point on, data has to be excluded?
Thank you so much in advance for ideas and answers.


No cake for spunky
I think you to base this on theory or you have to create your own theory about this. In time series there are ways to tell when a structural break has occurred, but I have no idea if it applies to your data. I would think you could test if different periods are statistically different from each other and test that with an f test although I am not certain that applies - its just a generic idea.