c Chart – Constant Subgroup Size
A c chart, or Count chart, is an attribute control chart that displays how the number of defects, or nonconformities, for a process or system is changing over time. The number of defects is collected for the same area of opportunity in each subgroup. The area of opportunity can be either a group of units or just one individual unit on which defect counts are performed. The c chart is an indicator of the consistency and predictability of the level of defects in the process.
When constructing a c chart, it is important that the area of opportunity for a defect be constant from subgroup to subgroup since the chart shows the total number of defects. When the number of items tested within a subgroup changes, then a u chart should be used, since it shows the number of defects per unit rather than total defects.
To create a C chart: • In Statit Express QC click on the “c” button on the menu bar. • In Statit Custom QC:
Select: QC Attribute charts C Chart The dialogs lead you through the creation of the charts.
If you have any difficulties, open the Help file using the Help button on the C Chart dialog, or select Help Contents and find “C Chart” in the Index.
