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

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20.5 Plotting the Number of Defects per Constant Unit of Measurement (C)

Consider the file VISITS_WEB.MTW already described in Chapter 16 (Capability analysis). This file contains the number of visits to a website during October and November 2010, specifying also the date and the weekday when those visits were produced.

The number of daily visits can be assumed to follow a Poisson distribution, and as the unit of measurement is constant (one day), a C chart can be used to represent its evolution.

Stat > Control Charts > Attributes Charts > C

nc20uf006.eps
nc20uf007.eps

The marked peak observed around day 10 is due to a newspaper advertisement of this website, clearly implying the presence of a special cause. Additionally, the so-called valleys, many of which also lie outside the limits, correspond to weekends when a remarkably smaller number of visits are produced in contrast to working days.

Using the Data options button, we remove the data corresponding to Saturdays and Sundays from the chart:

nc20uf008.eps

Then, the following chart is obtained, where the limits were recomputed based only on the remaining points. ...

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