Developed by Walter A. Shewhart at Bell Labs (Economic Control of Quality of Manufactured Product, 1931).
Plots a measure over time against limits set from its own stable history, separating routine noise from signals that deserve investigation.
When to reach for it
When managers react to every rise and dip in a metric, or a real shift in a process could be missed among the normal swings.
How it runs
- Pick the process measure.
- Gather a stretch of data from a stable period.
- Compute the mean and the standard deviation.
- Plot the data with limits three standard deviations out.
- Leave points inside the limits alone; investigate those outside.
- Add run rules to catch subtler patterns.
Where it sits in SOLVE eX
Go to the source
- Economic Control of Quality of Manufactured Product, 1931