Data Analysis with Microsoft Excel: Updated for Office 2007

504 Statistical Methods

You might have noticed that the range chart identifi es as out of control a

point that was apparently in control in the x chart but does not identify any

of the four observations that are out of control in the x chart. This is a com-

mon occurrence. For this reason, the x chart and range charts are often used

in conjunction to determine whether a process is in control. In practice, the

x chart and range chart often appear on the same page because viewing both

charts simultaneously improves the overall picture of the process. In this

example, you would judge that the process is out of control with both charts

but on the basis of different observations.

You can close the Coats Control Chart workbook now, saving your

changes.

The C Chart

Both the x chart and the range chart measure the values of a particular vari-

able. Now let’s look at an attribute chart that measures an attribute of the

process. Some processes can be described by counting a certain feature,

such as the number of fl aws in a standardized section of continuous sheet

metal or the number of defects in a production lot. The number of accidents

in a plant might also be counted in this manner. A C chart displays control

limits for the counts attribute. The lower and upper control limits are

LCL 5 c 23 "c

UCL 5 c 13 "c

where c is the average number of counts in each subgroup. If the LCL is

less than zero, by convention it will be set to equal zero, because a negative

count is impossible.

C Chart Example: Factory Accidents

The Accidents workbook contains the number of accidents that occurred

each month during a period of a few years at a production site. Let’s create

control charts of the number of accidents per month to determine whether

the process is in control.

To open the Accidents workbook:

1 Open the Accidents workbook from the Chapter12 folder.

2 Save the workbook as Accidents Control Chart. See Figure 12-11.