A statistical process control (SPC) tool that plots the number of defects in each sampled unit over time. You can use the U chart for processes where the measurement system is only capable of counting the number of defects in a sampled unit. The U chart is an industry standard for monitoring/ controlling process outputs over time and can be used for samples that are different sizes (e.g. different sized units or different sized lots of like sized units).
|When to Use||Purpose|
|Pre-project||Assist in project selection by identifying the process steps that exhibit high defect rates, unstable variation, or other symptoms pointing 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 the project is complete.|
|Post-project||Monitor output of the improved process after the project is complete.|
Discrete numeric Y (number of defects), size of inspected unit (for example, square inches of surface or words of text).