A control chart is a tool for detecting variation from special causes by monitoring the behaviour of a process characteristic over time.
A control limit is not a tolerance
This is the most common mistake in practice, and its consequences are serious.
A tolerance is the acceptance limit set by the customer or the design. A control limit is calculated from the process’s own data and shows the range of the process’s normal behaviour.
Replacing control limits with tolerance limits completely eliminates the chart’s ability to produce signals. Even if the process is drifting, the chart gives no warning as long as it stays within tolerance.
Chart selection
The right question is not “which chart should I use” but “what am I trying to control.” The joint AIAG & VDA SPC manual published on 1 July 2026 introduced a framework that classifies charts by control purpose.
The reliability of the chart depends on the reliability of the measurement system; in charts set up without MSA, the signals may be coming from the measurement rather than the process.
Chart selection, establishing control limits and interpreting signals are practised on real data sets in the SPC training. Which characteristic a chart is applied to is determined in the Control Plan training.