The Busy Tailor

When it was his turn to explain his recent work, Student Leaf stood up and portrayed an elegant experiment that used a central composite design with four factors. Master Stem asked, "Is this process ready for such an experiment?"

Student Leaf replied, "I do not understand."

Master Stem looked at him with an air of amusement. "If this process is not ready for an experiment, then you will make yourself very busy for no good reason."

"How can I tell, Master Stem, if a process is ready?"

"Have you computed a control chart for this process? Do you know if the process is in control?"

"I have not computed a control chart, but I do know that the process is too variable. I want to run an experiment to reduce that variation."

"I have a tailor I would like you to meet. He makes all the clothes for my family. I brought my oldest child in for a fitting and the tailor made measurements and started sewing. When I visited the next time, I had my youngest child with me. I apologized, but the tailor still insisted on doing the fitting. This required ripping out all the old seams, remeasuring and resewing. 'I am almost done with the clothes for your youngest child,' he told me, 'please come back tomorrow.' So I returned the next day, but this time I was accompanied by my middle child. 'No matter,' replied the tailor, 'I will rip out all the seams again and make the clothes fit your middle child.'"

"That is a very foolish tailor, Master Stem."

"And you, too, are foolish if you run an experiment without looking at the control chart first. If your process is out of control, that tells you that your process is not a single process, but is many instead. And you do not know which process is visiting at any time. Your experiment, carefully optimized for one process, will fit poorly for the other processes."

Creative Commons License This work is licensed under a Creative Commons Attribution 3.0 United States License. It was written by Steve Simon and was last modified on 2017-06-15. Category: Teaching resources