ISO 5725-5:1998 download free.Accuracy (trueness and precision) of measurement methods and results — Part 5: Alternative methods for the determination of the precision of a standard measurement method.
0.1 This part of ISO 5725 uses two terms trueness and precision to describe the accuracy of a measurement method. Trueness refers to the closeness of agreement between the average value of a large number of test results and the true or accepted reference value. Precision refers to the closeness of agreement between test results.
0.2 General consideration of these quantities is given in ISO 5725-1 and so is not repeated here. This part of ISO 5725 should be read in conjunction with ISO 5725-1 because the underlying definitions and general principles are given there.
0.3 ISO 5725-2 is concerned with estimating, by means of interlaboratory experiments, standard measures of precision. namely the repeatability standard deviation and the reproducibility standard deviation. It gives a basic method for doing this using the uniform-level design. This part of ISO 5725 describes alternative methods to this basic method.
a) With the basic method there Is a risk that an operator may allow the result of a measurement on one sample to influence the result of a subsequent measurement on another sample of the same material, causing the estimates of the repeatability and reproducibility standard deviations to be biased. When this risk is considered to be serious, the split-level design described in this part of ISO 5725 may be preferred as it reduces this risk.
b) The basic method requires the preparation of a number of identical samples of the material for use in the experiment. With heterogeneous materials this may not be possible, so that the use of the basic method then gives estimates of the reproducibility standard deviation that are inflated by the variation between the samples. The design for a heterogeneous material given ii this part of ISO 5725 yields information about the variability between samples which is not obtainable from the basic method; it may be used to calculate an estimate of reproducithty from which the between-sample variation has been removed.
c) The basic method requires tests for outhers to be used to identify data that should be excluded from the calculation of the repeatability and reproducibity standard deviations. Excluding outliers can sometimes have a large effect on the estimates of repeatability and reproducibility standard deviations, but in practice, when applying the outlier tests, the data analyst may have to use judgement to decide which data to exclude. This part of ISO 5725 describes robust methods of data analysis that may be used to calculate repeatability and reproducibility standard deviations from data containing outliers without using tests for outliers to exclude data, so that the results are no longer affected by the data analyst’s judgement.
1 Scope
This part of ISO 5725
— Provides detailed dascnptions of alternatives to the base method for determining the repeatability and reproducibility standard deviations of a standard measurement method, namely the split-level des1gn and a design for heterogeneous materials;
— describes the use of robust methods for analysing the results of precision experiments without using outlier tests to exclude data from the calculations, and In particular, the detailed use of one such method.
This part of ISO 5725 complements ISO 5725-2 by providing alternative designs that may be of more value in some situations than the basic design given in ISO 5725-2, and by providing a robust method of analysis that gives estimates of the repeatabity and reproducibity standard deviations that are less dependent on the data analysts judgement than those given by the methods described in ISO 5725-2.
2 Normative references
The following standards contain provisions which, through reference in this text, constItute provisions of thés part of ISO 5725. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this part of ISO 5725 are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.
ISO 3534-1:1993, Statistics — Vocabulary and symbols — Part 1: ProbabNity and general statistical terms. ISO 3534-3:1985, Statistics — Vocabulary and symbols — Part 3: Design of experiments.
ISO 5725-1:1994, Accuracy (trueness and precision) of measurement methods and results — Part 1: General pnnqoles and definitions.
ISO 5725-2:1994, Accuracy (trueness and precision) of measurement methods and resufts — Part 2: Basic method for the determination of repeatability arid reproducibility of a standard measurement method.
4.1.1 The uniform level design described wi ISO 5725-2 requires two or more identical samples of a material to be tested in each participating laboratory and at each level of the experiment. With this design there is a risk that an operator may allow the result of a measurement on one sample to influence the result of a subsequent measurement on another sample of the same material. If this happens, the results of the precision experiment will be distorted: estimates of the repeatability standard deviation a, will be decreased and estimates of the between- laboratory standard deviation m, will be increased. In the split-level design, each participating laboratory is provided with a sample of each of two sirrilar materials, at each level of the experiment, and the operators are told that the samples are not identical, but they are not told by how much the materials differ. The split-level design thus provides a method of determining the repeatability and reproducibility standard deviations of a standard measurement method in a way that reduces the risk that a test result obtained on one sample will influence a test result on another sample in the experiment.
4.1.2 The data obtained at a level of a split-level experiment may be used to draw a graph in which the data for one material are plotted against the data for the other, similar, matenal. An example is given in figure 1. Such graphs can help identify those laboratories that have the largest biases relative to the other laboratories. This is useful when it is possible to investigate the causes of the largest laboratory biases with the aim of taking corrective action.
4.1.3 It is common for the repeatability and reproducibility standard deviations of a measurement method to depend on the level of the material. For example, when the test result is the proportion of an element obtained by chemical analysis, the repeatability and reproducibility standard deviations usually increase as the proportion of the element increases. It is necessary, for a split-level experiment, that the two similar matenals used at a level of the experiment are so similar that they can be expected to give the same repeatability and reproducibility standard deviations. For the purposes of the split-level design, it is acceptable if the two materials used for a level of the experiment give almost the same level of measurement results, and nothing is to be gained by arranging that they differ substantially.
In many chemical analysis methods, the matrix containing the constituent of interest can influence the precision, so for a split-level experiment two materials with similar matrices are required at each level of the expenmerit. A sufficiently similar material can sometimes be prepared by spiking a material with a small addition of the constituent of interest. When the material is a natural or manufactured product, it can be difficult to find two products that are sulilcientty similar for the purposes of a split-level experiment: a possible solution may be to use two batches of the same product. It should be remembered that the object of choosing the materials for the split-level design is to provide the operators with samples that they do not expect to be identical.