CLAS requirements for proficiency testing

CLAS Requirements Document 7, March 2020

1. Introduction

  1. 1.1 Proficiency testing (PT) is an important tool to demonstrate laboratory competence and to assist in maintaining the quality of laboratory performance. The process is designed to verify the measurement capabilities of applicant and CLAS-certified laboratories. Each quantity in the laboratory's scope is subject to one or more of the following proficiency testing techniques, unless decided otherwise by CLAS; e.g., the required resources are not available, or the proficiency test would not be conclusive.
  2. 1.2 CLAS requirements for proficiency testing follow the requirements published in ILAC P9, ILAC Policy for Participation in Proficiency Testing Activities. This publication is available at
  3. 1.3 It is the responsibility of applicant and CLAS-certified laboratories to verify their calibration capabilities on an on-going basis through PT to meet the requirements of CLAS and ISO/IEC 17025. Laboratories are encouraged to seek out and to participate in PT programs offered by recognised PT providers. CLAS may nominate laboratories to participate in international PT activities.

2. Policy

  1. 2.1 CLAS certified and applicant laboratories shall prepare and document a PT participation plan. This plan shall describe what type of PT the laboratory participates in and how frequently the laboratory participates in PT. The plan shall take into account:
    • All of the types of devices on the laboratory scope of accreditation.
    • The range of accredited values on the laboratory scope of accreditation.
    • The laboratory measurement uncertainty. Larger measurement uncertainty relative to the type of device being calibrated may result in a reduced requirement for PT.
    • Similar types of devices and measurement techniques.
    • Previous participation in PT.
    • Alternative means of assessing and monitoring performance.
  2. 2.2 CLAS applicant laboratories shall demonstrate satisfactory participation in PT, where PT is available and appropriate, before accreditation is granted.
  3. 2.3 Laboratories shall regularly review their PT plan with respect to changes in staffing, methodology, instrumentation, etc.
  4. 2.4 CLAS assessment teams will review the laboratory PT plan for suitability and review the PT results at each assessment.

3. Description of PT Activities

  1. 3.1 Measurement Audit by Traceability
    1. 3.1.1 A PT scheme that is limited to situations where a single participant is tested independently is called a measurement audit.
    2. 3.1.2 This process of measurement audit uses the laboratory's reference, transfer, and other appropriate standards. CLAS conducts a measurement capability verification using the standards (reference, transfer, and other standards whose values are within the laboratory's scope) of the laboratory being verified. Laboratories must have their reference, transfer, or other appropriate standards calibrated by a higher-echelon laboratory, that satisfies the traceability criteria of the CLAS program. This verification is an on-going process and is performed when the laboratory has its standards calibrated by a higher-echelon laboratory.
    3. 3.1.3 Laboratories are required to provide CLAS with two sets of values for these standards; the first is the value of the standard before it was sent for calibration and the second is the value of the standard after it was returned from calibration.
    4. 3.1.4 These standards must be calibrated by NRC or another laboratory acceptable to CLAS (see CLAS Requirements Document 9). By prior arrangement, reference laboratories (i.e. higher-echelon laboratory) shall forward to CLAS a copy of the calibration certificate and not provide the calibration results to their client until notified by CLAS.
    5. 3.1.5 The laboratory being evaluated must provide CLAS with a calibration certificate or report similar to the ones that would normally be provided to a typical client. The content of the report must be according to CLAS Requirements Document 6, CLAS Requirements for Calibration Certificates. The certificate shall include the value of the standard and a statement of uncertainty. This statement must include a numeric value for the uncertainty. This value should not be smaller than the value of uncertainty in the Calibration and Measurement Capability (CMC) for which the laboratory was certified by CLAS.
  1. 3.2 Measurement Audit by Artefact
    1. 3.2.1 A PT scheme that is limited to situations where a single participant is tested independently is called a measurement audit.
    2. 3.2.2 This technique uses artefacts supplied by CLAS to represent, as much as possible, the typical measurement situation faced by the laboratory. Artefacts may be selected to verify difficult measurement situations such as the extreme ends of ranges.
    3. 3.2.3 The reference laboratory (i.e. higher-echelon laboratory) characterizes the appropriate parameters of the artefact under appropriate conditions and CLAS sends it to the laboratory being verified. The measurement results are not disclosed to the participants until the end of the measurement audit. CLAS provides a series of instructions for the laboratory to follow, including a schedule for the calibration of the artefact. The laboratory must follow these instructions and maintain the schedule unless approval has been received from CLAS to deviate from it. The laboratory must calibrate the artefact and provide CLAS with a calibration certificate (see CLAS Requirements Document 6).
    4. 3.2.4 Once the laboratory has completed its measurements, the artefact is returned to CLAS using the most appropriate method of transportation. Then the reference laboratory may be asked to repeat the measurements of the artefact to confirm its stability.
  1. 3.3 Proficiency Testing by Interlaboratory Comparison
    1. 3.3.1 CLAS certified laboratories may be asked to participate in international interlaboratory comparisons acceptable to CLAS.
    2. 3.3.2 CLAS may accept the results of other interlaboratory comparisons that meet the essential requirements of ISO/IEC 17043, General requirements for proficiency testing, and CLAS.

4. Analysis of PT results

For measurement audits, the measurement results are analyzed by CLAS at the time of the measurement audit based on the guidelines from ISO/IEC 17043, General requirements for proficiency testing. The analysis is designed to produce three possible conclusions:

  1. The measurement results are in agreement with the laboratory's scope, when the absolute value of the normalized error, En, is less than or equal to 0.5;
  2. The results are in doubt and must be further analyzed, when En is greater than 0.5 and less than or equal to 1; and
  3. The results are not in agreement with the laboratory's scope, when En is greater than 1. In this case an investigation shall be carried out to resolve the discrepancy and identify appropriate corrective action.

For PT, other than measurement audits, CLAS assesses the results at the time of the assessment visit, and/or when the results become available to CLAS, based on the criteria above.

5. Shipping of PT artefacts

Unless otherwise specified, each party is responsible for the cost of insurance and shipping of the artefact(s) to the next destination; i.e. to another participating laboratory or to CLAS.