718 CHAPTER 14 Statistical Process Control 12. Li-ion Batteries Rechargeable lithium ion batteries are used for mobile computing devices and portable power systems. Consider batteries manufactured to provide a capacity of 2200 mAh (milliampere hour) with a minimum of 2150 mAh. Batteries not meeting those specifications are defects. Listed below are numbers of defects in batches of 300 randomly selected batteries in each of 12 consecutive days of production. What action should be taken? 435812711997619 13. np Chart A variation of the control chart for p is the np chart, in which the actual numbers of defects are plotted instead of the proportions of defects. The np chart has a centerline value of np, and the control limits have values of np + 31npq and np - 31npq. The p chart and the np chart differ only in the scale of values used for the vertical axis. Construct the np chart for Example 1 “Defective Aircraft Altimeters” in this section. Compare the np chart to the control chart for p given in this section. 14-2 Beyond the Basics Chapter Quick Quiz 1. What are process data? 2. What is the difference between random variation and assignable variation? 3. Identify three specific criteria for determining when a process is out of statistical control. 4. What is the difference between an R chart and an x chart? In Exercises 5–8, use the following two control charts that result from testing batches of newly manufactured aircraft altimeters, with 100 in each batch. The original sample values are errors (in feet) obtained when the altimeters are tested in a pressure chamber that simulates an altitude of 6000 ft. The Federal Aviation Administration requires an error of no more than 40 ft at that altitude. 5. Is the process variation within statistical control? Why or why not? 6. What is the value of R? In general, how is a value of R obtained? 7. Is the process mean within statistical control? Why or why not? 8. What is the value of x? In general, how is a value of x found?
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