spctoolkit

by Donald J. Wheeler

The New Definition of Trouble

Shewhart's charts define when a process is in
trouble and will warn you so that you can take action.

Product 1411 was in trouble. After batches 1 through 29 ran without a problem, Batch 30 failed in the customer's plant. Batch 31 worked, but the next three batches failed. When the preliminary investigation exonerated the customer's process as the problem source, the pressure shifted to the supplier.

The supplier had a five-step process spanning three plants. Product 1411 started as a fluid produced by Step A in Plant One. The fluid was then shipped 400 miles to Plant Two, where Step B was performed. From there, it was shipped to Plant Three, where steps C, D and E were completed.

The supplier knew that, at no time, had any marginal material been passed on to the next step-all of the material supplied to Step E had been within specification. Therefore, the investigation focused on Step E as the likely problem source. While they investigated Step E, batches 35 through 52 were produced. Of these 18 batches, 10 worked, seven failed in the customer's plant and one was recycled because it did not meet internal specifications.

After much effort, the investigators became convinced that the problem did not originate in Step E. They then widened the investigation to include Step D. While Step D was scrutinized, batches 53 through 65 were produced and shipped to the customer. Of these 13 batches, only two worked.

After Batch 65, they decided to look at Step C. Finally, seven months after the problem arose, steps C and D were also eliminated as the problem source. By this time, they had produced 75 batches. However, only one of the last 10 batches had worked in the customer's plant.

Extending the investigation to Step B required the involvement of personnel from Plant Two. Even with the expanding number of people and the mounting number of man-hours, nothing was found at Step B. By now, eight months had passed, and only 18 of the last 54 batches had worked. So the team returned to Plant One and Step A.

As the investigators looked over the records for Step A, they found that the levels of "Compound M" had increased, beginning with Batch 30. The specification for Compound M was "not more than 1,500 ppm." This value was based upon the safe-handling requirements. Because the level of Compound M was fixed at Step A, and because safety requirements prohibited shipping any liquid that was out-of-spec, Compound M was not even measured at the subsequent steps.

Batches 1 through 29 averaged 78 ppm of Compound M, with levels ranging from 0 to 167 ppm. All of these batches worked.

Batches 30 through 51 averaged 191 ppm of Compound M, with levels ranging from 0 to 346 ppm. Of these 22 batches, only the 10 with less than 200 ppm worked.

Batches 53 through 85 averaged 412 ppm of Compound M, with levels ranging from 0 to 969 ppm. Of these 32 batches, only the eight with less than 200 ppm worked.

Dozens of people had worked for eight months to discover something that a simple control chart at the first production step could have revealed immediately-Plant One had ceased to produce a consistent product with Batch 30. This deterioration was not noticed because the specification was so much greater than the actual levels.

They are still trying to assess the damage done to the customer of Step E because of the performance degradation at Step A.

Conformance to specifications is no longer the definition of world-class quality. Specifications seldom take into account customer needs

Shewhart's charts, on the other hand, are the voice of the process. They define when a process is in trouble, and they will warn you so that you can take action to avoid even greater trouble.

Statistical control is an achievement attainable only by the persistent and effective use of Shewhart's charts. And those who can't use Shewhart's charts are doomed.

About the author

Donald J. Wheeler is an internationally known consulting statistician and the author of Understanding Variation: The Key to Managing Chaos and Understanding Statistical Process Control, Second Edition. © 1996 SPC Press Inc. Telephone (423) 584-5005.