HKSM

On a control chart, the upper and lower boundaries placed three standard deviations above and below the process mean (centerline) for a normally distributed measure. They show the range of variation expected from common causes and are not the same as specification limits.

Key Points

• Set at plus or minus three standard deviations around the centerline of the control chart.
• Indicate the expected common-cause variation of a stable, normally distributed process.
• Points beyond these limits signal potential special causes and a process out of control.
• Different from specification limits, which reflect requirements or tolerances, not process behavior.

Example

A project team tracks weekly defect counts. The average is 12 defects with a standard deviation of 2. The control limits are set at 12 minus 6 (LCL = 6) and 12 plus 6 (UCL = 18). A week with 19 defects falls outside the UCL, prompting investigation into a special cause such as a recent code change.

PMP Example Question

Which statement best describes control limits on a control chart?

1. They are the customer-defined tolerance boundaries for acceptable results.
2. They are fixed limits at two standard deviations from the mean for any process.
3. They are statistically derived bounds, typically at plus or minus three standard deviations from the mean, indicating expected process variation.
4. They are thresholds set by the project manager to meet schedule objectives.

Correct Answer: C — Statistically derived bounds at about plus or minus three sigma around the mean

Explanation: Control limits are based on process data and show the expected common-cause variation. They are not customer tolerances (specification limits) or arbitrary manager-set thresholds.