Ch17_257-263 - 72106 CH17 GGS 3/30/05 2:36 PM Page 257 C H...

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257 T EACHING S UGGESTIONS Teaching Suggestion 17.1: Japan’s change in status since WWII. Remind students that Japan began a few decades ago with perhaps the world’s worst quality and that “Made in Japan” was synony- mous with shoddy products just 45 years ago. Teaching Suggestion 17.2: Four interesting quotes from QC expert Philip Crosby. 1. “The cost of quality is the expense of doing things wrong.” 2. “There is absolutely no reason for having errors or de- fects in any product or service.” 3. “If quality isn’t ingrained in the organization, it will never happen.” 4. “It is much less expensive to prevent errors than to re- work, scrap, or service them.” Teaching Suggestion 17.3: Natural vs. assignable variations. Random chance l “natural” SpeciFc cause l assignable Teaching Suggestion 17.4: Mean and range charts. Mean and range charts tell us what we need to know about the process. Each plays a necessary role. A LTERNATIVE E XAMPLES Alternative Example 17.1: Twenty-Fve engine mountings are sampled each day and found to have an average width of 2 inches, with a standard deviation of 0.1 inch. To set control limits that in- clude 99.7% of sample means ( Z 5 3), 5 2.06 inches 5 1.94 inches Alternative Example 17.2: Several samples of size n 5 8 have been taken from today’s production of fencing poles. The average pole was 3 yards in length and the average sample range was 0.015 yard. We Fnd 99.7% control limits for the process below. A 2 5 0.373 from Table 17.2 LCL yards x XAR 52 5 2 5 2 3 0 373 0 015 2 994 .( .). UCL yards x 51 5 1 5 2 3 0 373 0 015 3 006 R 5 0 015 . yard X 5 3 yards LCL xx XZ 5 2 s 23 0 1 2 5 20 0 6 (./ ) . UCL 5 1 s 0 0 6 ) . Alternative Example 17.3: The average range of a process is 10 pounds. The sample size is 10. Using Table 17.2, D 4 5 1.78, D 3 5 0.22. Alternative Example 17.4: Based on samples of 20 IRS audi- tors, each observed handling 100 Fles, we Fnd that the total number of mistakes made in handling Fles is 220. We set 95.45% limits on this process below: 100 is the size of each sample Alternative Example 17.5: There have been complaints that the sports page of the Dubuque Register has lots of typos. The last six days have been examined carefully, and the number of typos/page recorded below. Is the process in control, using Z 5 2? LCL pp pZ 5 2 5 s 011 2 003 005 . ( )( . ) . UCL 5 1 5 s 017 . ( )( . ) . s p 5 2 5 (. ) ( . ) . 011 1 011 100 003 p 55 5 total no. mistakes total no. files 220 100 20 () ( ) . LCL pounds R DR 5 3 022 10 22 ( ) . UCL pounds R 5 4 178 10 178 . All days are in control. S OLUTIONS TO D ISCUSSION Q UESTIONS AND P ROBLEMS 17-1. The central limit theorem allows us to use the normal curve regardless of the distribution of the population we are trying to control. LCL or c cc 5 2 2 2 5 2 1 58 0 66 0 . ) . ( ) UCL c 5 5 2 1 5 8 5 6 6 . ) . c 15 6 2 5 /. 17 CHAPTER Statistical Quality Control Day Number of Typos Mon. 2 Tues. 1 Wed. 5 Thurs. 3 Fri. 4 Sat. 0 a 72106 CH17 GGS 3/30/05 2:36 PM Page 257
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258 CHAPTER 17 S TATISTICAL Q UALITY C ONTROL 17-2. The ultimate goal of - and R -charts is to ascertain, by a sampling procedure, that the process is kept within speciFed upper and lower bounds. The combination of and R -charts allows one
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Ch17_257-263 - 72106 CH17 GGS 3/30/05 2:36 PM Page 257 C H...

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