Tools › Quality › Control › SPC and Capability › Example
Montgomery Example 7.3: c chart of nonconformities on printed circuit boards
| Tool | SPC and Capability |
| Source | Montgomery, D. C. (2013). Introduction to Statistical Quality Control, 7th ed. Wiley, Hoboken, NJ. Example 7.3, Table 7.7. |
| Level | Undergraduate |
Problem
Twenty-six samples of 100 printed circuit boards were inspected and the total number of nonconformities in each sample recorded; the 26 samples held 516 nonconformities altogether. Compute the trial centre line and three-sigma limits of the c chart.
Inputs to Enter
- Chart type c (each line = count of nonconformities)
- 26 lines from Table 7.7 with Σc_i = 516
Answers
- c̄ = 516/26 = 19.85
- UCL = c̄ + 3 √c̄ = 33.22
- LCL = c̄ − 3 √c̄ = 6.48
Notes
The test script checks the limits from the total of 516 in 26 samples (the book rounds c̄ to 19.85 before taking the square root); paste the 26 counts of Table 7.7 to see the chart and the two out-of-control samples the book discusses. Select the c chart; not the default case.