Quality and Reliability Methods - SAS

Chapter 5 Shewhart Control Charts 85
Shewhart Control Charts for Attributes
Shewhart Control Charts for Attributes
In the previous types of charts, measurement data was the process variable. This data is often continuous,
and the charts are based on theory for continuous data. Another type of data is count data, where the
variable of interest is a discrete count of the number of defects or blemishes per subgroup. For discrete count
data, attribute charts are applicable, as they are based on binomial and Poisson models. Since the counts are
measured per subgroup, it is important when comparing charts to determine whether you have a similar
number of items in the subgroups between the charts. Attribute charts, like variables charts, are classified
according to the subgroup sample statistic plotted on the chart:
Table 5.1 Determining Which Attribute Chart to Use
Each item is judged as either conforming or
non-conforming
For each item, the number of defects is counted
Shows the number of
defective items
Shows the proportion
of defective items
Shows the number of
defective items
Shows the average
number of defective
items
NP-chart P-chart C-chart U-chart
• P-charts display the proportion of nonconforming (defective) items in subgroup samples which can vary
in size. Since each subgroup for a P-chart consists of N i items, and an item is judged as either
conforming or nonconforming, the maximum number of nonconforming items in a subgroup is N i .
• NP-charts display the number of nonconforming (defective) items in subgroup samples. Since each
subgroup for a NP-chart consists of N i items, and an item is judged as either conforming or
nonconforming, the maximum number of nonconforming items in subgroup i is N i .
• C-charts display the number of nonconformities (defects) in a subgroup sample that usually, but does
not necessarily, consists of one inspection unit.
• U-charts display the number of nonconformities (defects) per unit in subgroup samples that can have a
varying number of inspection units.
Note: To use the Sigma column property for P or NP charts, the value needs to be equal to the proportion.
JMP calculates the sigma as a function of the proportion and the sample sizes.
Note: For attribute charts, specify the defect count or defective proportion as the Process variable. The data
will be interpreted as counts, unless it contains non-integer values between 0 and 1.