The SPC Procedure
Constructing Charts for Proportion Nonconforming (p Charts)
You produce control charts for proportion nonconforming by using the PCHART statement.
The following notation is used in this section:
p | Expected proportion of nonconforming items that the process produces |
Proportion of nonconforming items in the ith subgroup | |
Number of nonconforming items in the ith subgroup | |
Number of items in the ith subgroup | |
Average proportion of nonconforming items taken across subgroups: | |
N | Number of subgroups |
Subgroup Summary Statistics
Each point on a p chart represents the observed proportion () of nonconforming items in a subgroup. For example, in Figure 20.7 the second subgroup contains 16 items, of which 2 are nonconforming. The summary statistic for the second subgroup is .
Figure 20.7: Proportions versus Counts
Note that an chart displays the number (count) of nonconforming items . You can use the NPCHART statement to create charts; for more information, see the section Constructing Charts for Number Nonconforming (np Charts).
Central Line
By default, the central line on a p chart indicates an estimate of p that is computed as . If you specify a known value () for p, the central line indicates the value of .
Control Limits
The control limits are computed as a specified multiple (k) of the standard error of above and below the central line. The default limits are computed using k = 3 (these are referred to as limits).
The lower and upper control limits, LCL and UCL, respectively, are computed as