Measurement Results Near a Limit: Agree the Decision Rule
Agree how measurement uncertainty is considered near an acceptance limit before supplier and customer compare inspection results.
China Thermic Editorial Team
When a measured result is close to an acceptance limit, supplier and customer need an agreed rule for deciding conformity. The numerical reading alone may not explain the risk associated with the decision. Define the measurement method and decision rule before inspection, rather than choosing a convenient interpretation after a borderline result appears.
START WITH THE REQUIREMENT
Identify the characteristic, units and applicable tolerance or performance limit. State the product condition in which the requirement applies. Confirm that both parties are interpreting the same drawing or specification revision. A disagreement about the requirement cannot be resolved merely by using a more detailed measurement report.
UNDERSTAND THE MEASUREMENT BASIS
A measurement result includes more than the displayed number. Consider the documented uncertainty relevant to the actual method and conditions. Calibration contributes important evidence, but an instrument's calibration certificate alone does not describe every influence in the production measurement. The setup, environment and procedure need appropriate consideration by competent measurement personnel.
AGREE HOW CONFORMITY IS DECIDED
Document how the acceptance decision accounts for uncertainty and the consequences of an incorrect decision. Where a guard band or another decision approach is specified, define it clearly and obtain the required agreement. This article does not prescribe a universal guard-band value or a replacement for contractual or applicable standard requirements.
KEEP BORDERLINE RESULTS TRACEABLE
Record the measured value, relevant method and decision applied. If a repeat measurement is authorized, retain the original observation and explain the reason for repeating it. Do not keep measuring until a preferred value appears. Where supplier and customer methods differ, investigate the differences before concluding that one party's product or measurement is necessarily wrong.
FREQUENT QUESTION: DOES TRACEABILITY GUARANTEE FITNESS FOR EVERY TOLERANCE?
No. A traceable measurement still needs uncertainty suitable for the intended decision. Traceability and fitness for a particular inspection task are related but different questions.
BEFORE PLACING AN ORDER
Ask for the proposed measurement method, reporting format and conformity decision rule for critical characteristics. Resolving these items before production reduces ambiguity when an actual result falls near the specified boundary.