Control Charts:

Control charts provide a way of objectively defining a process and variation. The intent of a control chart is to determine if a process is statistically stable and then to monitor the variation of stable process where activities are repetitive.

There are two types of variation:
1. Common or random causes of variation
These are inherent in every system over time, and are a part of the natural operation of the system. Resolving common cause problems requires a process change.
2. Special causes of variation
These are not part of the system all the time. They result from some special circumstance and require changes outside the process for resolution.

Common causes of variation are typically due to many small random sources of variation. The sum of these sources of variation determines the magnitude of the processes inherent variation due to common causes. From the sum, the process control limits and current process capability can be determined. Accepted practice uses a width of three standard deviations around the population mean (μ ± 3δ) to establish the control limits.

A process containing only common causes of variation is considered stable, which implies that the variation is predictable within the statistically established control limits. Processes containing special as well as common causes of variation are referred to as unstable processes.



Control charts are suitable for tracking items such as:
• Production failures
• Defects by life cycle phase
• Complaint/failures by application/software
• Response time to change request
• Cycle times/delivery times
• Mean time to failure