What Is Proficiency Testing?

Proficiency testing (PT) determines the performance of individual laboratories for specific tests or measurements
and monitors their ongoing measurement capability. It’s a key part of laboratory quality assurance, and it’s
required by most accreditation bodies under ISO/IEC 17025 for labs, and performed by providers accredited to
ISO/IEC 17043 — like NAPT.

How it Works?

Also called an interlaboratory comparison (ILC), this approach circulates a test artifact — a gage block, weight
set, thermocouple, or other reference item depending on discipline — among participating laboratories. Each
laboratory:

• Receives the test artifact from NAPT or the current participant in sequence
• Performs the measurement according to the round’s written instructions
• Reports its result and stated measurement uncertainty to NAPT
• Forwards the artifact to the next participant, or returns it to NAPT

Reference Values

Each participant’s results are scored against a reference value established through accredited reference laboratories and other participants. NAPT’s technical advisors select these reference labs based on demonstrated competence and measurement uncertainty capability — the same standard NAPT has applied for nearly 30 years running PT and ILC programs.

The En-Value

NAPT evaluates results using the En-value calculation defined in ISO 13528, which compares a participating laboratory’s reported measurement against the reference value while accounting for both parties’ stated uncertainties.

The En-value formula: En = (Vlab − Vref) ÷ sqrt(Ulab² + Uref²)

Where:

• Vlab = the laboratory’s measured value
• Vref = the reference value
• Ulab = the laboratory’s expanded uncertainty (k=2)
• Uref = the reference value’s expanded uncertainty (k=2)

Success Criteria

A result passes when: −1 ≤ En ≤ 1An En-value between −1 and 1 is a passing result. It means your lab’s measurement agrees with the reference value once both stated uncertainties are accounted for (k=2, approximately 95% coverage probability).

An En-value outside that range (|En| > 1) doesn’t automatically mean something is wrong with your lab — it means the result needs a closer look. NAPT flags it in your report, and the most common root causes are: measurement error or a procedural slip, an uncertainty budget that’s too tight for the measurement being made, equipment due for calibration or adjustment, or environmental conditions not fully accounted for.

Same facts and threshold as before, just reframed to sound less like a fail-state and more like a diagnostic step. The file in your NAPT Marketing folder is updated.

Why This Matters at NAPT?

NAPT is the only non-profit, ANAB-accredited PT provider under ISO/IEC 17043:2023, with 30 years running ILC/PT programs. Every En-value NAPT reports is calculated under ISO 13528 — the same statistical rigor a lab would otherwise have to build and validate in-house.