Quality and Reliability Methods - SAS

86 Shewhart Control Charts Chapter 5
Shewhart Control Charts for Attributes
P- and NP-Charts
Example: NP-Charts
The Washers.jmp data in the Quality Control sample data folder contains defect counts of 15 lots of 400
galvanized washers. The washers were inspected for finish defects such as rough galvanization and exposed
steel. If a washer contained a finish defect, it was deemed nonconforming or defective. Thus, the defect
count represents how many washers were defective for each lot of size 400. To replicate this example, follow
these steps or submit the JSL script below:
• Choose the Analyze > Quality And Process > Control Chart > NP command.
• Choose # defective as the Process variable.
• Change the Constant Size to 400.
• Click OK.
Control Chart(Sample Size(400), KSigma(3), Chart Col( :Name("# defective"),
NP));
The example here illustrates an NP-chart for the number of defects.
Figure 5.12 NP-Chart
Example: P-Charts
Again, using the Washers.jmp data, we can specify a sample size variable, which would allow for varying
sample sizes.
Note: This data contains all constant sample sizes. Follow these steps or submit the JSL script below:
• Choose the Analyze > Quality And Process > Control Chart > P command.
• Choose Lot as the Sample Label variable.
• Choose # defective as the Process variable.
• Choose Lot Size as the Sample Size variable.