Control Charts
Control Charts are a powerful statistical tool that may have many different applications. For example, they may be used to monitor key product variables and process parameters. They may also be used in the maintenance of process control and in the identification of special and common causes of variation. In addition, they may also be used for process improvement by showing the effects of process of change.
Difficulties associated with Control Charts:
- Poor understanding of the meaning of the chart
- Ill defined purpose of the chart
- Chart used is not suitable for the application
- Uncertainty of motives for using the chart
- Insufficient understanding of motives
- Inappropriate measurements made, including method of sampling
- Lack of support in analysing the data
- Lack of understanding of when to use control charts
- Not adequately training those who will be involved
- Failure to react to the plotted data trends
In addition, people get confused between “control” and “capability”. A process can be in control and not capable. "Control" means that the process is stable. "Capable" means “within specification”. People also focus on “product” and not “process”. Control Charting is a process improvement tool not a quality control or conformance to specification tool.
Overcoming the difficulties:
- Define the purpose of the chart
- Involve the operators at all stages
- State the aims of the chart clearly
- Define the data measurement criteria
- Define how the data are to be collected and by whom
- Constantly ask if data collection is adding value to the process, procedure or business
- Ensure that expert support is available during data collection, analysis and interpretation
- Ensure procedures are in place to cover the changes that result from the charting exercise
- Determine terminology and adhere to it
