Okay, I think there is a possible way to do this. You can create three aggregated items, 'Threshold Q1', 'Threshold Q2', 'Threshold Q3'. These would just aggregate AHT, but instead of Sum or Average you would define each one to use a different quartile cutoff (Q1, median, Q3 I think are the correct options).
Then you can create a new calculated item something like this:
if AHT < Threshold Q1, then return 'Q1'
else if AHT < Threshold Q2, then return 'Q2'
else if AHT < Threshold Q3, then return 'Q3'
else return 'Q4'
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