ITTC Decision Rule for the Conformity of Test Results


A new term was introduced in ISO/IEC 17025:2017: Decision Rule.

 

The definition of "Decision Rule" is: “rule that describes how measurement uncertainty is accounted for when stating conformity with a specified requirement”. Obviously, Decision Rule is related to the application of measurement uncertainty.

 

ITTC has established the document of the decision rule according to the requirement of ISO/ IEC 17025:2017. When no decision rule is demanded by the regulations, standards, specifications and customers while a statement of conformity is required, the statement of conformity will be reported according to ITTC decision rule.

 

. General Process for Selecting Decision Rules (Figure 1)

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. ITTC Decision Rule for the Conformity of Test Results

 

For non-numerical results, a statement of conformity can be reported directly according to the measured result and the specified limit. For numerical results, a statement of conformity shall be reported based on the expanded uncertainty under the 95% coverage probability. A numerical type value means that expresses a quantity and can conduct numerical calculation. The influence of expanded uncertainty on numerical results can be divided into the following ten cases (Figure 2) :

 

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    1) Case 1 and 6: When the measured value is extended up or down by the expanded uncertainty and the results all fall in green areas, a statement of meeting the requirements of the specification is declared, expressed by "Pass".

    

    2) Case 5 and 10: When the measured value is extended down or up by the expanded uncertainty and the results all fall in orange areas, a statement of not meeting the requirements of the specification is declared, expressed by "Fail".

    

    3) Case 2, 3, 4, 7, 8 and 9: When the measured value is extended down or up by the expanded uncertainty and the results fall in both green and orange areas, laboratory is unable to determine the conformity of the result, expressed by "No conclusion".

 


 

 

. The followings are two examples for your better understanding:


a) Non-numerical results: Taking the result of screw bending test as an example.

According to the requirements of the specification, the results meet the requirements of the specification if the screw does not fracture after screw bending test. Otherwise the requirements of the specification for the screw fractures after screw bending test are not met.

1) Case 1: meet the requirements of the specification for the screw does not fracture after test, expressed by "Pass".

2) Case 2: not meeting the requirements of the specification for the screw fractures after test, expressed by "Fail".

 

b) Numerical results: Determining the conformity of the results of formaldehyde content in leather (Figure 3)

Formaldehyde content in leather should not exceed 75 mg/kg according to the specification.

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As shown in Figure 3, assuming that the expanded uncertainty U is 3mg/kg with 95% coverage probability, the following statement of conformity can be reported:

1) Case 1: meet the requirements of the specification, expressed by "Pass".

For example: The formaldehyde content measured in laboratory is 71 mg/kg. And the result interval is (68--74) mg/kg combined with the expanded uncertainty, all below the limit. The conclusion will be expressed by "Pass".

 

2) Case 2, 3 and 4: laboratory is unable to determine the conformity of the result, expressed by "No conclusion".

For example: Like case 3, the formaldehyde content measured in laboratory is 75 mg/kg. And the result interval is (72--78) mg/kg combined with the expanded uncertainty, part of the values below the limit and part of the values above the limit. The conclusion will be expressed by "No conclusion".

 

3) Case 5: not meeting the requirements of the specification, expressed by "Fail".

For example: The formaldehyde content measured in laboratory is 80 mg/kg. And the result interval is (77--83) mg/kg combined with the expanded uncertainty, all above the limit. The conclusion will be expressed by "Fail".

 



. If results of multiple samples are involved for one test method, comprehensive evaluation shall be reported on the first page of the report besides evaluating each measured value as follows:

 

1)    All measured values meet the prescribed limit, expressed by "Pass".

 

2)    All measured values do not meet the prescribed limit, expressed by "Fail".

 

3)    All measured values cannot be determined in conformance to the specification, expressed by "No conclusion ".

 

4)    One or multiple measured values do not meet the prescribed limit, expressed by "Partially failed".

 

5) The conformance of one or multiple measured values cannot be determined but other results are in line with the stated limit, expressed by "Partially no conclusion ".

 

 

To summarize, we should determine the conformity by the interval where the results are located combined with the expanded uncertainty at 95% coverage probability instead of the point of measured value for numerical results. The define and use of decision rule will enable any user of the report to understand how measurement uncertainty is taken into account when a laboratory draws a conformity conclusion and make results more scientific and transparent.

 


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