Reading charts and reports
Control chart
- Reading charts and reports
Definition
A time series chart with a centre line at the process average and upper and lower control limits, usually three standard deviations away, used to tell normal variation from unusual events.
Also called
Statistical process control chart, SPC chart
For your role
Choose your role to see what this term means in your work.
Why it matters
It stops people reacting to random noise and shows when a change is real and worth investigating.
Where it is used
Quality management, operations monitoring and process improvement. Used by quality engineers, operations managers, process improvement leads and data analysts.
How to calculate it
Centre line = mean of the measurements
Control limits = mean ± 3 × sigma (for single values, sigma is often estimated as average moving range ÷ 1.128)
Worked example
Raket Parts B.V. received an average of 154.6 sales orders a week over 38 weeks of 2025. The average week-to-week change was 21.0 orders, giving limits of 98.8 and 210.4. Every week fell inside the limits, the highest being 206 and the lowest 107, so the weekly swings are normal variation and no single week needs investigating.
Raket Parts B.V. is a fictional company created for Insyt.
Definitions vary. Different chart types (individuals, X-bar and R, p and c charts) and rules for signals are used. Check how your organisation calculates it.