Industrial Statistics with Minitab

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

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:

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