Measurement Just Outside Tolerance

Calibration Troubleshooting

Follow a controlled review when a result sits outside tolerance. Check the method, instrument, uncertainty, acceptance rule, and product impact.

Contents

• A Measurement Is Just Outside Tolerance. Should You Reject the Part?
• Record the actual result
• Check the tolerance boundary
• Find the applicable decision rule
• Check the measurement system
• Consider measurement uncertainty
• Separate measurement from disposition
• Document the decision
• Borderline measurement checklist
• Continue the investigation

A Measurement Is Just Outside Tolerance. Should You Reject the Part?

A measurement is slightly outside the specified tolerance. The instrument has a current calibration record. The result is near the acceptance limit.

Check how the measurement was made before accepting or rejecting the part. Also, see which decision rule applies.

A borderline measurement needs evidence. The display number is just one part of the decision.

Record the Actual Result

Start with the measurement that caused the concern.

Record:

• Measured value
• Instrument identification
• Measurement method
• Measurement location
• Applicable specification
• Stated tolerance or acceptance requirement

Keep the original result. Don’t replace it with a second measurement just because it’s easier to accept. If you repeat the measurement, record the new result and the conditions.

Check the Tolerance Boundary.

Confirm the actual requirements before deciding on conformity.

Check the drawing, specs, customer needs, standards, or procedures for acceptance limits.

Also, ensure you’re using the correct revision of the requirement.

Check any result that looks "just outside" a limit. Compare it to the documented requirement, not an assumed limit.

Find the Applicable Decision Rule

The decision rule shows how measurement results and uncertainty guide conformity decisions.

Don’t create a universal acceptance formula for every situation.

The decision rule can come from a specification, customer need, standard, or a documented procedure.

If you can't find a decision rule, pause the conformity decision. Then, check which requirement governs the inspection.

Check the Measurement System.

A borderline result is a good reason to review the measurement system.

Check:

• Instrument identification
• Calibration status
• Relevant calibration results
• Measurement range
• Measurement method
• Positioning and alignment
• Reference surfaces
• Contact conditions
• Relevant environmental conditions

Check the measurement method or setup first. If it wasn't controlled, don't treat the result as reliable.

For more on measurement investigation, read: Your Instrument Is Calibrated. Why Can the Measurement Still Be Wrong?

Consider Measurement Uncertainty

Measurement uncertainty can affect results close to a specification boundary.

VIM defines measurement uncertainty as a non-negative value. It shows how quantity values change for a measurand. This is based on the information we have.

Consider relevant uncertainty information according to the applicable measurement procedure and decision rule.

Don’t assume uncertainty makes an out-of-tolerance result acceptable or requires rejection. The decision rule determines how the result is evaluated.

JCGM VIM: Measurement Uncertainty

Separate Measurement from Disposition

The measurement result provides evidence. The disposition decision applies the relevant requirement and decision rule to that evidence.

These steps should not be treated as the same. If a result is close to a tolerance limit, check the measurement method and uncertainty first. Then, make a final decision.

The decision-maker should have the required information from the relevant quality procedure.

Document the Decision

A borderline result should leave a clear record.

Document:

• Original measurement
• Instrument used
• Calibration status
• Measurement method
• Applicable requirement
• Relevant uncertainty information
• Decision rule
• Final conformity decision
• Any further investigation performed

The record should help a qualified person. It should show what was measured, which requirement was used, and why the final decision was made.

Borderline Measurement Checklist

1. Record the original measurement.
2. Identify the instrument used.
3. Confirm calibration status.
4. Review relevant calibration information.
5. Confirm the measurement range.
6. Review the measurement method and setup.
7. Confirm the applicable specification.
8. Confirm the correct tolerance or acceptance requirement.
9. Identify the applicable decision rule.
10. Consider measurement uncertainty.
11. Separate the measurement result from the disposition decision.
12. Document the evidence and final decision.

Continue the Investigation

If the instrument has a current calibration certificate and the measurement is still unexpected, check:

If the calibration certificate is good, but the part didn’t pass inspection, read: Your Calibration Certificate Passed. Why Is the Part Still Failing Inspection?

If two calibrated instruments show different readings, be alert: Two Calibrated Instruments Show Different Readings. This discrepancy could indicate a problem that needs investigation. Which One Should You Trust?

If the measurement issue continues after recalibration, read: The Instrument Was Recalibrated, but the Measurement Problem Remains.

Related calibration guides

• Instrument Failure or Process Drift? A Practical Investigation
• Can Calibration Data Help Investigate Process Drift?
• Why a Calibrated Instrument Can Give the Wrong Result
• Why Inspection Can Fail After an Instrument Passes Calibration