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?
- Control chart
- Run chart
- Pareto chart
- 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.