812 APPENDIX D 13. Except for a different vertical scale, the basic control chart is identical to the one given for Example 1. Chapter 14: Quick Quiz 1. Process data are data arranged according to some time sequence. They are measurements of a characteristic of goods or services that result from some combination of equipment, people, materials, methods, and conditions. 2. Random variation is due to chance, but assignable variation results from causes that can be identified, such as defective machinery or untrained employees. 3. There is a pattern, trend, or cycle that is obviously not random. There is a point lying outside the region between the upper and lower control limits. There are at least eight consecutive points all above or all below the centerline. 4. An R chart uses ranges to monitor variation, but an x chart uses sample means to monitor the center (mean) of a process. 5. No. The R chart has at least eight consecutive points all lying below the centerline, there are at least eight consecutive points all lying above the centerline, there are points lying beyond the upper and lower control limits, and there is a pattern showing that the ranges have jumped in value for the more recent samples. What a mess! 6. R = 67.0 ft. In general, a value of R is found by first finding the range for the values within each individual subgroup; the mean of those ranges is the value of R. 7. No. The x chart has a point lying beyond the upper control limit, and there are at least eight consecutive points lying below the centerline. 8. x = -2.24 ft. In general, a value of x is found by first finding the mean of the values within each individual subgroup; the mean of those subgroup means is the value of x . 9. No. The control charts can be used to determine whether the mean and variation are within statistical control, but they do not reveal anything about specifications or requirements. 10. Because there is a downward trend, the process is out of statistical control, but the rate of defects is decreasing, so we should investigate and identify the cause of that trend so that it can be continued. 7. The process appears to be out of statistical control because of a downward trend, but the number of defects appears to be decreasing, so the process is improving. Causes for the declining number of defects should be identified so that they can be continued. 9. The process is out of statistical control because there are points lying beyond the upper control limit and there are points lying beyond the lower control limit. The graph shows a “sawtooth” pattern because presidential elections held every four years tend to attract many more voters than the national elections that do not include a presidential election. 11. Based on the control chart for p it appears that the process is within statistical control. However, the numbers of defects in batches of 100 are significantly high. Although the process is within statistical control, immediate corrective action should be taken to reduce the large numbers of defects.
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