Data Analysis with Microsoft Excel: Updated for Office 2007

506 Statistical Methods

Each point on the C chart in Figure 12-12 represents the number of

accidents per month. The average number of accidents per month was 7.11.

Only in the seventh month did the number of accidents exceed the upper

control limit of 15.12 with 16 accidents. Since then, the process appears

to have been in control. Of course, it is appropriate to determine the spe-

cial causes associated with the large number of accidents in the seventh

month. In the case of this fi rm, the workload was particularly heavy during

that month, and a substantial amount of overtime was required. Because

employees put in longer shifts than they were accustomed to working, fa-

tigue is likely to have been the source of the extra accidents.

You can close the Accidents Control Chart workbook, saving your results.

The P Chart

Closely related to the C chart is the P chart, which depicts the proportion of

items with a particular attribute, such as defects. The P chart is often used to

analyze the proportion of defects in each subgroup.

Let p denote the average proportion of the sample that is defective. The

distribution of the proportions can be approximated by the normal distribu-

tion, provided that np and n^112 p^2 are both at least 5. If p is very close to 0

number of

accidents per

month exceeded

control limits

Figure 12-12
C chart
of the number
of accidents
per month