The Six Sigma Handbook by Thomas Pyzdek & Paul A. Keller

Author:Thomas Pyzdek & Paul A. Keller
Language: eng
Format: epub
Publisher: McGraw-Hill
Published: 2010-03-25T16:00:00+00:00

FIGURE 8.9 Control chart patterns: freaks.

FIGURE 8.10 Control chart patterns: drift.

Cycles often occur due to the nature of the process. Common cycles include hour of the day, day of the week, month of the year, quarter of the year, week of the accounting cycle, and so on (Fig. 8.11). Cycles are caused by modifying the process inputs or methods according to a regular schedule. The existence of this schedule and its effect on the process may or may not be known in advance. Once the cycle has been discovered, action can be taken. The action might be to adjust the control chart by plotting the control measure against a variable base. For example, if a day-of-the-week cycle exists for shipping errors because of the workload, you might plot shipping errors per 100 orders shipped instead of shipping errors per day. Alternatively, it may be worthwhile to change the system to smooth out the cycle. Most processes operate more efficiently when the inputs are relatively stable and when methods are changed as little as possible.

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