# Process capability for a qualified WPC tolerance

Canonical: https://woodplasticcomposite.net/guides/process-capability-for-a-qualified-wpc-tolerance/
Language: en
Updated: 2026-10-15
Version: 1
Review: AI-assisted English editorial draft. Human technical review has not been recorded.

## Summary

A capability index is meaningful only when its assumptions and data basis are appropriate. It should not be used as a decorative quality score without checking the process being summarised.

## Technical context

The relationship between process variation and specification limits depends on stability, measurement quality and the distribution of results. Combining unrelated production states can produce a misleading single index.

https://woodplasticcomposite.net/guides/process-capability-for-a-qualified-wpc-tolerance/#technical-context

## What to measure and document

Define the characteristic, sampling period and product revision. Establish that the measurement system is suitable and examine process behaviour before interpreting capability. State the calculation method and any assumptions rather than publishing an unexplained number.

https://woodplasticcomposite.net/guides/process-capability-for-a-qualified-wpc-tolerance/#verification-plan

## Implications for development and specification

Use capability evidence to improve production and inform realistic tolerances. It is not a replacement for agreed acceptance procedures or finished-product qualification. Buyers should ask what the reported figure covers and whether it represents the material they will receive.

https://woodplasticcomposite.net/guides/process-capability-for-a-qualified-wpc-tolerance/#practical-decision

## Related reading and evidence limits

General engineering-statistics guidance; not a WPC product standard. Use the linked work as related reading. The practical checks above are editorial recommendations, not reported test results or a substitute for product qualification.

https://woodplasticcomposite.net/guides/process-capability-for-a-qualified-wpc-tolerance/#reading-scope

## Key points

The relationship between process variation and specification limits depends on stability, measurement quality and the distribution of results.
Define the characteristic, sampling period and product revision.
Use capability evidence to improve production and inform realistic tolerances.

## Comparison

Review step | Evidence to retain
Material and geometry | Exact specimen or product identity
Verification | Method, conditions and individual observations
Decision | Approved scope and documented limitations

## What evidence is needed for process capability for a qualified WPC tolerance?

Define the characteristic, sampling period and product revision. Establish that the measurement system is suitable and examine process behaviour before interpreting capability. State the calculation method and any assumptions rather than publishing an unexplained number.

## Sources

1. NIST engineering statistics handbook — https://www.itl.nist.gov/div898/handbook/