What is Proficiency Testing?

Definition

Proficiency testing determines the performance of individual laboratories for specific tests or measurements and monitors their ongoing capability. It is a key component of laboratory quality assurance and is typically required by accreditation bodies.

How It Works

Also called interlaboratory comparison, this approach distributes test artifacts (such as ring gages or gage blocks) among participating laboratories. Each laboratory:

• Receives the test artifact
• Performs measurements according to specified instructions
• Reports results along with measurement uncertainty to the administrator
• Returns the artifact or forwards it to the next participant

Reference Values

Results are compared against reference values, typically established through accredited reference laboratories. These laboratories are selected by technical advisors based on demonstrated competence and measurement uncertainty capabilities.

The E_n-Value

Results are evaluated using the E_n-value calculation, which compares a participating laboratory's reported measurement against the reference value while accounting for stated uncertainties.

The E_n-value formula is:

Where:

• V_lab = laboratory's measured value
• V_ref = reference value
• U_lab = laboratory's expanded uncertainty (k=2)
• U_ref = reference value's expanded uncertainty (k=2)

Success Criteria

A result passes when:

This indicates that the laboratory's measurement agrees with the reference value within the stated uncertainties (k=2, approximately 95% coverage probability).

Results outside this range (|E_n| > 1) require investigation and corrective action by the laboratory. Common causes include:

• Measurement errors or procedural issues
• Underestimated measurement uncertainty
• Equipment calibration problems
• Environmental factors not properly accounted for