Testing of industrial products - Aggregates for construction

Results of the cross-testing experiment on the Los Angeles test

Repeatability and reproducibility

Estimates of repeatability and reproducibility are given in the Tables. The standard deviations are also shown in the Figure. From the Figure it may be seen that it is reasonable to summarise the estimates using the linear functional relationships given in the Tables. These relationships could be quoted in the European Standard test method as a statement of the precision of the Los Angeles test.

The ASTM description of the Los Angeles test method (ASTM, 1989) states that the repeatability coefficient of variation is 2.0%: similar to the corresponding values in the Tables. The reproducibility coefficient of variation given in the ASTM test method is 4.5%: this is somewhat smaller than the corresponding values in the Tables, suggesting that there may be scope for the European laboratories to improve their reproducibility.

The Figure shows that the repeatability of the Los Angeles test depends on the level of the results. However, inspection of the Histograms suggests that most laboratories should be able to obtain results of duplicate tests that differ by no more than 1.0 (LA units), with any of the relatively homogeneous materials used in this experiment. In practice, Los Angeles test results are usually reported to the nearest integer, so it will be convenient to have a repeatability limit that is also an integer.

Assessment of the precision of the Los Angeles test

It has been argued (J»rck, Sym and Powell, 1994) that the reproducibility standard deviation of a mechanical test, when expressed as a coefficient of variation, should be no more than about 8%, if the test method is to be used to assess the compliance of aggregates with specifications. The results in the Tables show that the reproducibility of the Los Angeles test is adequate when assessed by this criterion.

The estimates of reproducibility are substantially larger than the corresponding values for repeatability (compare values of r1 and R1 in the Tables). Also, there are a number of laboratories that give standardised laboratory biasses that are either all positive or all negative (see the Mandel plots). These observations indicate that there are one or more factors that are having a detrimental effect on the reproducibility. Comparisons of equipment and procedures, and cross-over experiments, should allow these factors to be identified so that the reproducibility of the test can be improved.

Although the repeatability of the test may be considered to be generally satisfactory, several laboratories reported between-test-portion ranges that were classed as stragglers or outliers, or that appear in the Histograms to be large in comparison with those obtained by most other laboratories. If laboratories do duplicate tests regularly, use the results to check their repeatability, and implement improvements in their procedures where necessary, they should learn how to prevent such happenings.

If a laboratory wishes to check their repeatability, they will find their results more informative if they round them to the nearest 0.1 (LA units).