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

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20.7 Plotting the Number of Defects per Variable Unit of Measurement (U)

Similarly to previous cases, using the data in file FABRIC.MTW, proceed to draw a U chart as follows.

Stat > Control Charts > Attributes Charts > U

nc20uf011.eps

Using all default options, one obtains:

nc20uf012.eps

Tests performed with unequal sample sizes

The process is under statistical control. The limits vary because the sample sizes also vary.

When the control limits are not constant, the values of the upper and lower limits shown by Minitab are the ones from the last sample.

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