Significance and Use
ASTM test methods are frequently intended for use in the manufacture, selling, and buying of materials in accordance with specifications and therefore should provide such precision that when the test is properly performed by a competent operator, the results will be found satisfactory for judging the compliance of the material with the specification. Statements addressing precision and bias are required in ASTM test methods. These then give the user an idea of the precision of the resulting data and its relationship to an accepted reference material or source (if available). Statements addressing determinability are sometimes required as part of the test method procedure in order to provide early warning of a significant degradation of testing quality while processing any series of samples.
Repeatability and reproducibility are defined in the precision section of every Committee D02 test method. Determinability is defined above in Section 3. The relationship among the three measures of precision can be tabulated in terms of their different sources of variation (see Table 1).
When used, determinability is a mandatory part of the Procedure section. It will allow operators to check their technique for the sequence of operations specified. It also ensures that a result based on the set of determined values is not subject to excessive variability from that source.
A bias statement furnishes guidelines on the relationship between a set of test results and a related set of accepted reference values. When the bias of a test method is known, a compensating adjustment can be incorporated in the test method.
This practice is intended for use by D02 subcommittees in determining precision estimates and bias statements to be used in D02 test methods. Its procedures correspond with ISO 4259 and are the basis for the Committee D02 computer software, Calculation if Precision Data: Petroleum Test Methods. The use of this practice replaces that of Research Report RR:D02–1007.
Standard practices for the calculation of precision have been written by many committees with emphasis on their particular product area. One developed by Committee E11 on Statistics is Practice E 691. Practice E 691 and this practice differ as outlined in Table 2.
TABLE 1 Sources of Variation
|Repeatability||Complete||Same||Same|| Same||Almost same|
|Determinability||Incomplete||Same||Same|| Same||Almost same|
| (Part result)|
TABLE 2 Differences in Calculation of Precision in Practices D 6300 and E 691
|Element||This Practice||Practice E 691|
|Applicability||Limited in general to homogeneous samples for which serious sampling problems do not normally arise.||Permits heterogeneous samples.|
|Number of duplicates||Two||Any number|
|Precision is written |
|Test method||Each sample|
|Outliers||Rejected, subject to subcommittee approval. ||Rejected if many laboratories or for cause such as blunder or not following method.|
| Retesting not generally permitted.|| Laboratory may retest sample having rejected data.|
|Rejection limit||20 %|| 5 %|
|Analysis of variance||Two-way, applied globally |
to all the remaining data
| One-way, applied to each sample separately.|
|Precision multiplier||t, where t is the two-tailed Student's t for 95 % probability.||2.8=1.96|
|Increases with decreasing laboratories × samples particularly below 12.||Constant.|
|Variation of precision with level||Minimized by data transformation. Equations |
for repeatability and reproducibility are generated in the retransformation process.
| User may assess from individual sample precisions.|
1.1 This practice covers the necessary preparations and planning for the conduct of interlaboratory programs for the development of estimates of precision (determinability, repeatability, and reproducibility) and of bias (absolute and relative), and further presents the standard phraseology for incorporating such information into standard test methods.
1.2 This practice is generally limited to homogeneous products with which serious sampling problems do not normally arise.
1.3 This practice may not be suitable for solid or semisolid products such as petroleum coke, industrial pitches, paraffin waxes, greases, or solid lubricants when the heterogeneous properties of the substances create sampling problems. In such instances, use Practice E 691 or consult a trained statistician.
2. Referenced Documents (purchase separately) The documents listed below are referenced within the subject standard but are not provided as part of the standard.
D123 Terminology Relating to Textiles
D3244 Practice for Utilization of Test Data to Determine Conformance with Specifications
E29 Practice for Using Significant Digits in Test Data to Determine Conformance with Specifications
E456 Terminology Relating to Quality and Statistics
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
ISO4259 Petroleum Products-Determination and Application of Precision Data in Relation to Methods of Test Available from International Organization for Standardization, 1 rue de Varemb, Case postale 56, CH-1211 Geneva 20, Switzerland.
interlaboratory; precision; repeatability; reproducibility; round robin; Bias; Cochran's test; Hawkins' test; Lubricants; Petroleum products; Precision--petroleum analysis; Regression analysis; Reproducibility; Transformations; Types ofdependence; Weighted linear regression analysis;
ICS Number Code 03.120.20 (Product and company certification. Conformity assessment); 75.080 (Petroleum products in general)
ASTM International is a member of CrossRef.
Citing ASTM Standards
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