Control Chart

A chart that plots process measurements over time against predefined upper and lower control limits, with a central line showing the process average to help spot trends or shifts toward either limit.

Key Points

  • Shows data over time with a centerline (mean/target) and upper/lower control limits (UCL/LCL).
  • Helps distinguish common-cause variation from special-cause signals.
  • Control limits are calculated from process performance, not customer specifications.
  • Used to monitor process stability and trigger investigation when rules are violated.

Example

A QA lead charts weekly defects per 1,000 lines of code. After several points trend upward and one exceeds the UCL, the team pauses releases to investigate a recent tool change causing the variation.

PMP Example Question

Which tool should a project manager use to determine whether a process is stable by tracking performance over time against statistically derived limits?

  1. Control chart
  2. Run chart
  3. Pareto chart
  4. Histogram

Correct Answer: A — Control chart

Explanation: A control chart includes a centerline and control limits to assess process stability; a run chart lacks control limits.

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