Notes on decision rule as per ISO/IEC 17025:2017 requirements
Introduction
The revised ISO/IEC 17025:2017 laboratory accreditation standard introduces a new concept, i.e., “risk-based thinking” which requires the operator of an accredited laboratory to plan and implement actions to address possible risks and opportunities associated with the laboratory activities, including issuance a statement of conformity to product specification or a compliance statement against regulatory limits.
The risk-based approach to management system implementation is one in which the breadth and depth of the implementation of particular clauses is varied to best suit the perceived risk involved for that particular laboratory activity.
Indeed, the laboratory is responsible for deciding which risks and opportunities need to be addressed. The aims as stated in the ISO standard clause 8.5.1 are:
- to give assurance that the management system achieves its intended results;
- to enhance opportunities to achieve the purpose and objectives of the laboratory;
- to prevent, or minimize, undesired impacts or interfering elements to cause failures in the laboratory activities, and
- to achieve improvement of the activities.
The decision rule as required in ISO/IEC 17025:2017
On the subject of decision rule for conformity testing, the word of ‘risk’ can be found in the following relevant clauses of this international standard:
Clause 7.1.3
“When the customer requests a statement of conformity to a specification or standard for the test or calibration (e.g. pass/fail, in-tolerance/out-of-tolerance), the specification or standard and the decision rule shall be clearly defined. Unless inherent in the requested specification or standard, the decision rule selected shall be communicated to, and agreed with the customer.”
Clause 7.8.6.1:
“When a statement of conformity to a specification or standard is provided, the laboratory shall document the decision rule employed, taking into account the level of risk (such as false accept and false reject and statistical assumptions) associated with the decision rule employed and apply the decision rule.”
Clause 7.8.6.2
“The laboratory shall report on the statement of conformity, such that the statement clearly identified:
- to which results the statement of conformity applies;
- Which specifications, standards or part therefor are met or not met;
- The decision rule applied (unless it is inherent in the requested specification or standard).”
From these specified requirements, it is obvious that clearly defined decision rules must be in place when the laboratory’s customer requests for inclusion of a statement of conformity on the specification in the test report after laboratory analysis. Therefore, the tasks in front of the accredited laboratory operator are how the decision rules are going to be for a tested commodity or product, based on the laboratory’s own measurement uncertainty estimated, and how to communicate and convince the customers on its choice of reporting limits against the given specification or regulatory limits when issuing such conformity statement.
Comments on: "Decision rule and ISO/ IEC17025:2017" (4)
How can we implement the decision rules(e.g pass, conditional pass, conditional fail and fail) in our evaluation of uncertianity calibration report.
Plz provide the %t of these rules if you can.
Thanx
Please see my email reply to you on this query.
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GH Yeoh
How can we implement the decision rules(e.g pass, conditional pass, conditional fail and fail) in our evaluation of uncertianity calibration report.
Plz provide the %t of these rules if you can.
(Please assist me on this as well, thank you)
The work of making a decision rule starts from what level of risk you would like to take by claiming a non-conformance when the actual situation is that it is within specification (so-called Type I error). Generally we consider a maximum 5% error or minimum 95% confidence to reject the correct situation. For example, let your actual measurement with uncertainty is X +/- U and the specification limit is S(Maximum). As your uncertainty U comes from “2 x standard uncertainty u” where 2 is a coverage factor for 95% confidence, the (X + U) is the uppermost limit with a 2.5% error (right-hand or one-sided tail of a normal probability distribution). If you check the normal distribution table, you will find that a 5% (alpha) error gives a coverage factor of 1.645, instead of 1.96 (or 2 as a whole number). Hence, the realistic critical value for you to claim conformance is (X + 1.645 x u). Any value below this critical value can be claimed as a PASS and any value between this critical value and your specification upper limit is to be claimed as “conditional pass”. When the test value crosses over the upper specification limit, you shall call it a FAIL. This is being considered a conservative approach to the decision rule, where the laboratory risk is kept to the minimum.