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Montgomery Example 7.3: c chart of nonconformities on printed circuit boards
| Tool | Pareto and Root Cause |
| Source | Montgomery, D. C. (2013). Introduction to Statistical Quality Control, 7th ed. Wiley, Hoboken, NJ. Example 7.3. |
| Level | Undergraduate |
Problem
Twenty-six samples of 100 printed circuit boards were inspected and the total number of nonconformities in each sample recorded. Set up a c chart, identify any out-of-control samples, and recompute the trial limits without them.
Inputs to Enter
- Counts by sample (loaded by the Example 7.3 selector entry as dated records, one per day): 21, 24, 16, 12, 15, 5, 28, 20, 31, 25, 20, 24, 16, 19, 10, 17, 13, 22, 18, 39, 30, 24, 16, 19, 17, 15
- Chart by day (c chart of the counts)
Answers
- c̄ = 516/26 = 19.85, UCL = 33.22, LCL = 6.48
- Samples 6 (5 nonconformities) and 20 (39) fall outside the limits
- Revised without them: c̄ = 472/24 = 19.67, UCL = 32.97, LCL = 6.36, all points in control
Notes
Choose the Montgomery Example 7.3 entry in the selector; the counts were taken from the IMSL C-chart example that cites the book.