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Problem Solving with the Box Plots

I was working with a client recently to solve problems with rework and scrap. One of the improvement teams manufactures a diecast cylinder about 3-inches long. The internal diameter varied from the top to the bottom. They took three sample measurements of internal diameter: top, middle and bottom. They asked me how I would illustrate the problem using QI Macros.

My immediate thought: Box and Whisker Plot. So, I selected their data in Excel and created a Box & Whisker Plot using QI Macros add-in:

Box and Whisker Plot created by QI Macros

I added a target line of .25 to the box whisker plot using QI Macros Chart Tools menu:

Analysis of the Box and Whisker Plot

This chart served as the problem statement for the improvement team. From this it was clear that:

• Die 1 was running too large
• Die 2 too low
• Die 3 was closest to the target

The engineers immediately set to work figuring out how to resolve the issues and move the product onto target.  They had to go back to the plant to test their countermeasures, but I could tell from their discussions that the problem would soon be solved.

How could a Box and Whisker Plot illustrate your quality problem?

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