Out of Control (OOC)

Introduction: OOC

Out of Control (OOC) is a concept from Statistical Process Control (SPC) used to describe a process that has become unstable due to abnormal causes. Unlike common cause variation, which reflects natural process fluctuations, OOC signals unusual disturbances that require corrective action.

Background

The term gained importance with the development of SPC methods by Walter A. Shewhart. Control charts, central to SPC, allow organisations to distinguish between normal variation and special causes that disrupt process stability. Recognising OOC situations prevents defective outputs and maintains reliable performance in manufacturing and service operations.

Key Elements/Features

• Indicators of OOC:
  • A point outside the control limits on a chart.
  • Two of three consecutive points near a control limit.
  • Continuous upward or downward trends.
• Process Stability: OOC means the process is no longer predictable.
• Quality Assurance: Timely detection and action are critical to preserving quality.

Applications/Examples

OOC signals are identified through SPC tools such as X-bar and R charts. In production, this might mean a machine drift causes defective parts. In services, a sudden spike in customer complaints may indicate OOC conditions. Detecting these signals ensures issues are corrected before they escalate.

Relevance/Impact

OOC situations require immediate investigation and corrective measures. Many organisations use an Out of Control Action Plan (OCAP) to respond quickly. An OCAP defines responsibilities, corrective steps, and timelines to restore control and prevent recurrence. This approach minimises disruption, maintains quality, and supports continuous improvement.

See also

• Statistical Process Control (SPC)
• Control Chart
• Common Cause Variation
• Out of Control Action Plan (OCAP)