A capability number can look decisive while concealing an unstable process. Understand the data-generating conditions before using a single index to support a purchasing decision.
ASQ's process-capability guidance relates capability analysis to process behaviour and specification requirements. ASQ — process capability.
Establish the basis
Confirm the measurement method, data period and process conditions. Ask whether the selected analysis fits the distribution and whether the process was sufficiently stable for the intended interpretation. Mixing different conditions can make one summary misleading.
Keep the requirement visible
Capability analysis compares process performance with a defined specification under stated assumptions. It does not create the specification or prove that every future unit will conform. Avoid adopting a universal acceptance threshold from an unrelated project.
A useful supplier discussion includes the method and supporting data, not just an index on a slide. Missing assumptions should be resolved before the figure is treated as evidence for a production commitment.
Source reviewed 25 October 2026. Practical interpretation and original illustrations by PLUSHIP / Qunhe; examples are illustrative.
Related reading
Process Spread and Process Centering Are Different Issues
The Check Stage of PDCA Needs Evidence of the Outcome
A Process Audit and a Product Evaluation Have Different Boundaries