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Appendix B: Computer Resources 363ContinuedFigure B.19 Minitab window showing the P Chart dialog box.The np Chart: Control Chart for Nonconforming Units. In an np chart we plotthe number of nonconforming units in an inspected sample. The np chart is verysimilar to the p chart except that in the p chart we plot the fraction of nonconformingunits in each inspected sample. Moreover, in the p chart the sample sizescould be equal or unequal whereas in the np chart the sample sizes are equal.Otherwise, both the p chart and np chart can be implemented under the samecircumstances. We summarize below some specific points that are important forthe np chart:• The inspection sample sizes should be equal.• The sample size should be large enough to include somenonconforming units.• Record the sample size, number of nonconforming (np) ineach sample and plot the number of nonconforming on thecontrol chart.• To construct an np control chart follow all the steps for thep control charts.The c Chart. In many situations, we are interested in studying the number ofnonconformities in a sample, which is also called the inspection unit, rather thanstudying the fraction nonconforming or total number of nonconforming in thesample. To construct the c chart first enter the data (number of nonconformities)in one column, say C1, of the Worksheet window. Then from the command
364 Part V: Appendicesmenu select Stat > Attributes Charts > C. The dialog box titled C Chart shown inFigure B.20 appears immediately. We illustrate the construction of a c chart withExample B.16.The u Chart. The u chart is essentially the c chart except that the u chart is alwaysbased on the number of nonconformities per inspection unit. In other words, theactual sample size may be other than one or may vary, but the control limits of theu chart are always determined based on one inspection unit. Thus, for example,if n is constant then one can use either a c chart or a u chart. We illustrate the constructionof a u chart with Example B.17.The u Chart—Variable Sample Sizes. The procedure for constructing a u chartwith variable sample size is the same as for the u chart with fixed sample size.EXAMPLE B.16A paper mill has detected that almost 90 percent of rejected paper rolls are due tononconformities of two types, that is, holes and wrinkles in the paper. The Six SigmaGreen Belt team in the mill decided to set up control charts to reduce or eliminate thesenonconformities. To set up control charts the team decided to collect some data bytaking a random sample of five rolls each day for 30 days and counting the number ofnonconformities (holes and wrinkles) in each sample. The data are shown in Table 19.7,page 287. Set up a c control chart using these data.Solution:1. Enter all the data (number of nonconformities given in Table 19.7 inChapter 19) in column C1 of the Worksheet window. Click on Stat in thecommand menu.2. Select Control Charts in the pull-down menu under the Stat command menu.3. Select Attributes Charts from the Control Charts command menu.4. Click C in the Attributes Charts command menu. The dialog box entitled C Chartshown in Figure B.20 appears immediately.5. Enter C1 in the box under Variables.6. In the dialog box entitled C Chart there are several options available such as Scaleand Labels. Thus, for instance, if you select the Label option a new dialog box willappear where you can enter the title of the c chart and any footnotes that youwould like to see on the output of the c chart and then click OK. By default thetitle will be such as C Chart for C1 or C Chart for “name of the variable” if youhave given such name (say nonconformities) in column C1 of the data window.Use the option C Chart Options, for example, if you want to specify the meanvalue of nonconformities in the population instead of estimating it using the givendata. Then, click OK in the C Chart dialog box. The desired c control chart willappear in the Session window. Thus, in our example the output of the c chart is asshown in Figure 19.13 of Chapter 19, page 287.Continued
- Page 391 and 392: Index TermsLinksDdata set, skew of
- Page 393 and 394: Index TermsLinksfinal inspection 16
- Page 395 and 396: Index TermsLinkshierarchical design
- Page 397 and 398: Index TermsLinkslaw of sines 33-34l
- Page 399 and 400: Index TermsLinksmicrometer caliper
- Page 401 and 402: Index TermsLinksoutput, action on 2
- Page 403 and 404: Index TermsLinksQQS-9000 standards
- Page 405 and 406: Index TermsLinkssampling 150-61adva
- Page 407 and 408: Index TermsLinkssurface finishgages
- Page 409 and 410: Index TermsLinksvariance 208variati
- Page 411 and 412: Appendix AQuality Inspector Certifi
- Page 413 and 414: 334 Part V: Appendices2. Measuring
- Page 415 and 416: 336 Part V: Appendices5. Verificati
- Page 417 and 418: 338 Part V: AppendicesComprehension
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- Page 423 and 424: 344 Part V: AppendicesEXAMPLE B.2Us
- Page 425 and 426: 346 Part V: Appendicesthe option Si
- Page 427 and 428: 348 Part V: AppendicesContinuedFigu
- Page 429 and 430: 350 Part V: Appendices(a) As shown
- Page 431 and 432: 352 Part V: AppendicesBar Chart. En
- Page 433 and 434: 354 Part V: AppendicesContinuedFigu
- Page 435 and 436: 356 Part V: AppendicesEXAMPLE B.11C
- Page 437 and 438: 358 Part V: AppendicesShewhart X -
- Page 439 and 440: 360 Part V: AppendicesContinuedTabl
- Page 441: 362 Part V: AppendicesThe p Chart:
- Page 445 and 446: 366 Part V: AppendicesContinued7. I
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- Page 451 and 452: 372 Part V: Appendices3. NOTES ON B
- Page 453 and 454: 374 Part V: AppendicesUnits Square
- Page 455 and 456: 376 Part V: AppendicesUnits of Capa
- Page 457 and 458: 378 Part V: AppendicesApothecaries
- Page 459 and 460: 380 Part V: AppendicesUnits of Area
- Page 461 and 462: 382 Part V: Appendices1 gallon (gal
- Page 463 and 464: 384 Part V: Appendices1 ton, gross
- Page 465 and 466: 386 Part V: AppendicesContinuedz.00
- Page 467 and 468: 388 Part V: AppendicesContinuedn D
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- Page 471 and 472: Appendix HSample Tables ofANSI/ASQ
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Appendix B: Computer Resources 363
Continued
Figure B.19 Minitab window showing the P Chart dialog box.
The np Chart: Control Chart for Nonconforming Units. In an np chart we plot
the number of nonconforming units in an inspected sample. The np chart is very
similar to the p chart except that in the p chart we plot the fraction of nonconforming
units in each inspected sample. Moreover, in the p chart the sample sizes
could be equal or unequal whereas in the np chart the sample sizes are equal.
Otherwise, both the p chart and np chart can be implemented under the same
circumstances. We summarize below some specific points that are important for
the np chart:
• The inspection sample sizes should be equal.
• The sample size should be large enough to include some
nonconforming units.
• Record the sample size, number of nonconforming (np) in
each sample and plot the number of nonconforming on the
control chart.
• To construct an np control chart follow all the steps for the
p control charts.
The c Chart. In many situations, we are interested in studying the number of
nonconformities in a sample, which is also called the inspection unit, rather than
studying the fraction nonconforming or total number of nonconforming in the
sample. To construct the c chart first enter the data (number of nonconformities)
in one column, say C1, of the Worksheet window. Then from the command