Victor,
I had a quick look at your thread and have a couple of suggestions.
First, in your case the LSL should probably always be zero. In healthcare, patients want the service ASAP, which is zero. In other industries, such as work I have done with restaurants, the LSL isnt always zero. That is, at “higher end” restaurants, the customer often appreciates a wait time to gather with friends and have a drink at the bar. At a fast food restaurant people want the order filled ASAP.
Second, another observation I have is that you seem to have four groups of customers (A, B, C, and D). And each group has more progressively higher standards. I would suggest setting a different USL for each group, in the same manner that a chemical company, or restaurant, or a tire store does. If you can identify the segment (group) you are servicing, then you know what their USL is. Then you must improve the processes offered to satisfy each.
Third, look at Petes advice on how to truly identify what the USL is for each of the four groups. His idea is a good one, one which is preferable over merely asking the customer before the experience.
And finally, if given only the information you present in your table, I would very subjectively set the USL at 30 minutes to hopefully satisfy up to 90% of the patients.
Hope this is of some value. As far as your professor goes, s/he apparently has their own way (formula) for determining the USL. All I can say for certain is that the LSL is zero. And don’t overlook what Pete said.
Regards,
Dr. Scott
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