500 Statistical Methods
A 2 accounts for both the factor of 3 from the earlier equations (used when s
was known) and for the fact that the average range represents a proxy for the
common-cause variation. (There are other alternative methods for calculating
control limits when s is unknown.) As you can see from the table, A 2 depends
only on the number of observations in each subgroup. Furthermore, the control
limits become tighter when the subgroup sample size increases. The most typi-
cal sample size is 5 because this usually ensures normality of sample means.
You will learn to use the control factors in the table later in the chapter.x Chart Example: A Coating Process
The data in the Coats workbook come from a manufacturing firm that
sprays one of its metal products with a special coating to prevent corrosion.
Because this company has just begun to implement SPC, s is unknown for
the coating process.To open the Coats workbook:1 Open the Coats workbook from the Chapter12 data folder.
2 Save the workbook as Coats Control Chart.
Figure 12-8 shows the contents of the workbook.Figure 12-8
The Coats
workbook