A statistical process control (SPC) tool that plots the proportion defective per sample or subgroup over time. You can use the NP chart for processes where the measurement system is only capable of determining whether a unit is defective or not defective. The NP chart is an industry standard for monitoring and controlling process outputs over time. The NP chart can be used for samples of different sizes; however, it is best suited for a constant sample size. For different sized samples, try the P chart instead.
|When to Use||Purpose|
|Pre-project||Assist in project selection by identifying process steps that exhibit high defect rates, unstable variation, or other symptoms that point to the need for improvement.|
|Start of project||Verify process stability when performing a baseline capability analysis.|
|Mid-project||Investigate effects of input variables on process output over time.|
|Mid-project||Verify process stability when performing confirmation runs after implementing improvements.|
|End of project||Verify process stability after implementing controls to obtain a final assessment of process capability.|
|End of project||Graphically compare the pre-project process dynamic behavior to the post-improvement dynamic behavior.|
|Post-project||Control inputs to the improved process after project is completed.|
|Post-project||Monitor output of the improved process after project is completed.|
Discrete numeric Y (number of defectives), number of units sampled per lot.