This guide can be used at any point in the development or improvement of a test method, if it is desired to pursue reduction of its variability.

There are three circumstances in which a subcommittee responsible for a test method would want to reduce test variability:

4.2.1 During the development of a new test method, ruggedness testing might reveal factors which produce an unacceptable level of variability, but which can be satisfactorily controlled once the factors are identified.

4.2.2 Another is when analysis of data from an interlaboratory test of a test method shows significant differences between levels of factors or significant interactions which were not desired or expected. Such an occurrence is an indicator of lack of control which means that the precision of the test method is not predictable.

4.2.3 The third situation is when the method is in statistical control, but it is desired to improve its precision, perhaps because the precision is not good enough to detect practical differences with a reasonable number of specimens.

The techniques in this guide help to detect a statistical difference between test results. They do not directly answer questions about practical differences. A statistical difference is one which is not due to experimental error, that is, chance variation. Each statistical difference found by the use of this guide must be compared to a practical difference, the size of which is a matter of engineering judgment. For example, a change of one degree in temperature of water may show a statistically significant difference of 0.05 % in dimensional change, but 0.05 % may be of no importance in the use to which the test may be put.