Revisione della norma ISO 4259 Andrea Gallonzelli

Transcription

1 Revisione della norma ISO 4259 Andrea Gallonzelli 1

2 ISO 4259, Petroleum products - Determination and application of precision data in relation to methods of test Editions: 1979 / Cor. 1:1993 / 2006 ISO/DIS :2015, Petroleum products - Precision of measurement methods and results - Part 1: Determination of precision data in relation to methods of test ISO/DIS :2015, Petroleum products - Precision of measurement methods and results - Part 2: Application of precision data in relation to methods of test 2

3 ISO/DIS 4259:2015 (Part 1 and Part 2) The committee responsible for this document is ISO/TC 28, Petroleum products and related products of synthetic or biological origin This edition cancels and replaces the third edition (ISO 4259:2006) The two parts encompass both the determination of precision estimates and the application of precision data. They combine the information in ASTM D6300 regarding the determination of the precision estimates and the information in ASTM D3244 for the utilization of test data A third part on monitoring and management of precision data in relation to methods of test is envisaged to be published at a later point in time 3

4 ISO/DIS :2015 This International Standard covers the methodology for the design of an Interlaboratory Study (ILS) and calculation of precision estimates of a test method specified by the study The procedures in this International Standard have been designed specifically for petroleum and petroleum related products, which are normally homogeneous. However, the procedures described in this International Standard can also be applied to other types of homogeneous products 4

5 ISO/DIS :2015 The stages in planning of an interlaboratory study for the determination of the precision of a test method are as follows: preparing a draft method of test planning and executing the pilot study planning and executing the interlaboratory study The pilot study is necessary to verify the robustness/ruggedness of the procedure and to estimate approximately the precision and bias of the test method (at least twelve laboratory/sample combinations shall be included) 5

6 ISO/DIS :2015 Planning and executing the interlaboratory study There shall be at least 6 participating laboratories, but it is recommended this number be increased to 8 or more in order to ensure the final precision is based on at least 6 laboratories and to ensure the precision statement is more representative of the user population The number of samples shall be sufficient to adequately represent the types of materials to which the test method is to be applied, to cover the range of the property measured at approximately equidistant intervals, and to give reliability to the precision estimates. If precision is found to vary with the level of results in the pilot study, than at least 5 samples shall be used in the interlaboratory study 6

7 ISO/DIS :2015 Planning and executing the interlaboratory study In order to correctly estimate precision versus level relationship, it is important that the choice of samples evenly covers the range and materials for the property measured, so that an estimated relationship is not too dependent upon the leverage of a sample with extreme property value It is strongly recommended that the leverage of each planned sample in the sample set design be assessed ASTM D7778, Standard Guide for Conducting an Interlaboratory Study to Determine the Precision of a Test Method 7

8 ISO/DIS :2015 Pre-screening of the results Examination of information returned by participants to determine compliance with agreed upon test protocol and method of test Identification of unusual or extreme results by GESD technique (Rosner test) ASTM D7915, Standard Practice for Application of Generalized Extreme Studentized Deviate (GESD) Technique to Simultaneously Identify Multiple Outliers in a Data Set 8

9 ISO/DIS :2015 Statistical treatment of the results Calculation of precision estimates over all samples Transformation of data (if necessary) Tests for outliers (Cochran and Hawkins) Identification of excessively influential sample(s) (Cook's Distance) Analysis of variance (two-factor with replication) Calculation of precision estimates (r & R) Evaluation of R / r ratio A large R/r ratio (i.e. > 4) is an indication that bias between laboratories is a dominant contributor towards R 9

10 This International Standard covers the methodology for the application of precision estimates of a test method derived from ISO In particular, it contains the procedures for setting the property specification limits based upon test method precision where the property is determined using a specific test method, and in determining the specification conformance status when there are conflicting results between supplier and receiver Application of repeatability and reproducibility Specifications Quality control against specifications Dispute procedure 10

11 Construction of specifications limits in relation to precision of the specified test method A 1 denotes the upper limit and A 2 denotes the lower limit The value chosen for a specification limit shall take into account the reproducibility of the test method adopted, as follows: for a double limit (A 1 and A 2 ), the specified range (stated or implied) shall be not less than four times the reproducibility R, i.e. (A 1 - A 2 ) 4R for a single limit (A 1 or A 2 ), the specified limit shall be a distance not less than twice the reproducibility, R, away from the implied limit, that is, if the upper implied limit is 100 %, then (100 - A 2 ) 2R, or if the lower implied limit is zero, then A 1 2R 11

12 Definition of scope of test methods in relation to precision It is recommended that the lower limit of the scope of the test method is not less than the region of the lowest value tested in the ILS, and is at least 2R greater than the lowest achievable result. Similarly, it is recommended that the upper limit of the scope of a test method is not greater than the region of the highest value tested in the ILS, and is at least 2R less than the highest achievable result 12

13 Definition of scope of test methods in relation to precision Sulfur in automotive fuels by WDXRF EN ISO 20884:2011 R = 0,1201 X + 1,9 Test method lower limit: X = 2R = 2 (0,1201 X + 1,9); X = 5 mg/kg Specification upper limit: A 1 5 mg/kg Comment: if the upper limit A 1 of a specification is set to 2R, the supplier cannot verify the compliance of the product as all conforming results (95 % confidence) fall out side the working range of the method (see next slides) 13

14 Quality control against specification Prerequisites for acceptance for laboratory test results: Each lab's test result shall be obtained from a test method that is in statistical control, as substantiated by in-house control charts or other equivalent statistical techniques (see ASTM D6299) The standard deviation from the control charts (or equivalent statistical techniques), as calculated from at least 30 most recent results obtained over at least 15 days, with no more than 2 results per day that are separated by at least 6 hours, shall not exceed the published test method standard deviation (R / 2,77) 14

15 Quality control against specification If evidence exists from the published results of multiple PTP, that the R for a published test method is statistically inconsistent with the R achieved, the latter may be used in lieu of the published R to judge compliance with this clause, provided all of the following conditions are met: if legally permissible upon mutual agreement between disputing parties, and the precision is estimated with at least 30 degrees of freedom 15

16 Quality control against specification Each lab shall be able to demonstrate, by way of results from participation in Proficiency Testing Program(s), if available, a sustained testing proficiency and a lack of bias relative to PTP averages for the appropriate test method(s). In the event that suitable PTP is not available, proficiency shall be demonstrated by way of testing Certified Reference Materials (CRM) and in-house control chart on quality control (QC) samples, or by other method validation techniques acceptable to both parties 16

17 Testing margin at the supplier A supplier who has no other source of information on the true value of a characteristic than a single result shall consider, with 95 % confidence, that the product meets the specification limit, only if the result, X, is such that: in the case of a single upper limit, A 1 : X A 1-0,59 R in the case of a single lower limit, A 2 : X A 2 + 0,59 R in the case of a double limit (A 1 and A 2 ), both these conditions are satisfied 17

18 Testing margin at the recipient A recipient who has no other source of information on the true value of a characteristic than a single result shall consider that the product fails the specification limit with 95 % confidence, only if the result, X, is such that: in the case of a single upper limit, A 1 : X > A 1 + 0,59 R in the case of a single lower limit, A 2 : X < A 2-0,59 R in the case of a double limit (A 1 and A 2 ), either of these conditions applies 18

19 Quality control against specification Research Octane Number EN ISO 5164:2014 EN 228:2012, single lower limit A 2 : 95,0 Calculation of test margins (single result, 95 % confidence): A 2-0,59 R A 2 A 2 + 0,59 R 94,6 95,0 95,4 Nonconformance zone for the recipient Conformance zone for the supplier 19

20 Dispute procedure Resolve dispute by negotiation Given the complexity of the procedure, it is highly recommended that supplier and recipient attempt to resolve the dispute through negotiation to arrive at mutually acceptable terms for settlement Dispute resolution procedure If the supplier and the recipient cannot reach agreement regarding the specification conformance for the quality of the product on the basis of their existing results, then the dispute resolution procedures given in either this clause or Annex B shall be adopted. In order to use Annex B, the degree of criticality, p c, has to be agreed upon in advance between supplier and recipient 20

21 Dispute procedure Stage I Each laboratory shall reject its original results and obtain at least three other acceptable results on their own check sample to ensure that the work has been carried out correctly under repeatability conditions. The average of the acceptable results in each laboratory shall then be computed, divergent results being discarded If the re-testing does not resolve the dispute, then: Product meets specification if: 21

22 Dispute procedure Stage I It cannot be stated with confidence whether the product does or does not comply with the specification limit if: In this case, resolution of the dispute may be achieved by negotiation If dispute whatever the difference 22

23 Dispute procedure Stage II ISO 4259:2006: In case of dispute, the two laboratories shall contact each other and compare their operating procedures and apparatus. Following these investigations, a correlation test between the two laboratories shall be carried out on their check samples. The average of at least three acceptable results shall be computed, in each laboratory, and these averages compared as in Stage I REMOVED!!! Proposal: If and the dispute is unresolved, the two laboratories shall contact each other and compare their operating procedures and apparatus 23

24 Dispute procedure Stage III If the disagreement on product conformance status still remains and resolution of the dispute cannot be reached by negotiation, at least one additional third-party laboratory that is neutral, expert and accepted by the two parties in dispute shall be invited to assist in the dispute resolution All parties shall first agree on a common adjudication sample. Subject to sample volume adequacy, each party shall obtain their own average to be used for adjudication purpose using at least three acceptable results for the adjudication sample. If there is insufficient amount of adjudication sample available, all parties shall agree to the number of test results that each party is to use to calculate their average to be used for adjudication purpose 24

25 Dispute procedure Stage III One additional third-party laboratory If the difference between the most divergent laboratory adjudication result and the average of the two other laboratory adjudication results is less than or equal to R 3 : 25

26 Dispute procedure Stage III One additional third-party laboratory If the difference between the most divergent laboratory adjudication result and the average,, of the other adjudication results is more than R 3, the following procedure shall be adopted: 26