Process Stability Test

  • Process Stability is the ability of the process to stay within the Control Limits & do not exhibit any special cause symptoms in the process.
  • If the process data points remain consistent over time, the outputs will fall within the Process range (Control Limits) & do not exhibit any special causes, and then the process is said to be Stable or in Control.
  • If the data points are spread beyond the control limits, and the test fails for any of the special cause tests, then the process is Unstable or Out of Control.

Basic Control Charts Types

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  • Control charts are used to test the Stability of the Process.

Control Limits

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  • Analyze and control process performance
  • Distinguish between special and common cause variation
  • Identify and prevent the process from special causes

Control Limit Vs Specification Limit

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  • It is ideal to have specification limits within control limits but not compulsory. UCL and LCL can be more than USL and LSL, respectively.
  • Control limits are calculated from the data and describes what the process is capable of achieving.
  • Specification limits are set by the customer, management, or process requirements and describes what customer should achieve.

Control Chart Examples

  • Variable charts - Continuous data
    - Cycle time
    - Lengths
    - Financial
    - Generally more information
    - Normally one characteristic per chart
    - Can be more expensive
  • Attribute chart - Discrete data
    - Pass/Fail
    - Good/Bad
    - Right/Wrong
    - Can be less information per chart
    - Generally for quality checks
    - Charts based on the Poisson distribution or binomial
    - Less expensive