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

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20.2 Plotting the Proportion of Defective Units (P)

Using the data in file MOTORS.MTW, proceed to construct a P chart as follows.

Stat > Control Charts > Attributes Charts > P

nc20uf001.eps
nc20uf002.eps

The control limits depend on the sample sizes. When the sample sizes vary, the limits also vary.

Since the number of daily motors produced is very similar, we can choose a constant subgroup size in subgroup sizes. In this case 1350 (approximately the average daily production) and consequently a chart with constant control limits would be obtained.

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