c and u control charts
Monitors the number of defects per sample (c) or per unit inspected (u), where one item can have several defects.
Menu: Graphs → More charts → Control chart: c, u…
When should I use it?
- Counting defects such as scratches per panel or errors per page.
What are the assumptions?
- Defects occur independently (Poisson); equal inspection areas for c.
How do I run it in StatLab?
- Choose the defect count (and the number of units for a u chart).
How do I read the output?
| Output | Meaning |
|---|---|
| Center line | Average defects (per unit for u). |
| Limits | c̄ ± 3√c̄, or ū ± 3√(ū/n). |
Worked example
On the practice survey dataset (File → Sample dataset), this command — which the dialog's Paste button also produces — runs the procedure:
PROC vizCChart /y=score /n='' /x='' /kind=c /rules=YES.
Output tables: Rule Violations.
Computed by StatLab's validated procedure code when this page was built. The example only illustrates the steps; choose variables that fit your own research question.
What are common mistakes?
- Using a c chart when the inspected amount varies (use u).