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Industrial Statistics with Minitab by Xavier Tort-Martorell Llabres, Lluis Marco Almagro, Pere Grima Cintas

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20.4 Plotting the Number of Defective Units (NP)

Using data in file CATHETER.MTW, proceed to construct a NP chart as follows.

Stat > Control Charts > Attributes Charts > NP

nc20uf003.eps

Using the default options one obtains:

nc20uf004.eps

One point lies above the upper control limit. A special cause was found for this unusual increase in the number of defective catheters (a defective lot of raw material), making it reasonable to recalculate the control limits after removing such a singular point.

nc20uf005.eps

After the recalculation, as expected, the upper control limit is now slightly smaller.

Although Minitab allows the use of variable sample sizes when constructing NP charts, that is not appropriate. The best practice to facilitate NP charts interpretation is to use a constant sample size, whenever possible. If a constant sample size cannot be used, it is better to draw a P chart instead.

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