ASTM D6300-08 - Page 2

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

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 for	Test method	Each sample
Outlier tests: Within laboratories Between laboratories	Sequential Cochran test Hawkins test	Simultaneous k-value h-value
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 at once.	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.