Division 3.1.1.3 Applicability/ruggedness (minor changes)
United Kingdom
Such studies use the deliberate introduction of minor reasonable variations by the laboratory and the observation of their consequences.
The pre-investigative studies have to be carried out by selecting factors of the sample pre-treatment, clean up and analysis, which may influence the measurement results. Such factors may include the analyst, the source and the age of reagents, solvents, standards and sample extracts, the rate of heating, the temperature, the pH-value as well as many other factors that may occur in the laboratory. These factors should be modified in an order of magnitude that matches the deviations usually encountered among laboratories.
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Identify possible factors that could influence the results.
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Vary each factor slightly.
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Conduct a ruggedness test using the approach of Youden (15)(16). (Other approved methods may be used at this point. The Youden approach, however, keeps the required time and effort to a minimum). The Youden approach is a fractional factorial design. Interactions between the different factors cannot be detected.
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Where a factor is found to influence the measurement results significantly, conduct further experiments to decide on the acceptability limits of this factor.
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Factors that significantly influence the results should be identified clearly in the method protocol.
The basic idea is not to study one alteration at a time but to introduce several variations at once. As an example, let A, B, C, D, E, F, G denote the nominal values for seven different factors that could influence the results, if their nominal values are changed slightly. Let their alternative values be denoted by the corresponding lower case letters a, b, c, d, e, f and g. This results in 27 or 128 different possible combinations.
It is possible to choose a subset of eight of these combinations that have a balance between capital and small letters (Table 11). Eight determinations have to be made, which will use a combination of the chosen factors (A-G). The results of the determinations are shown in Table 11 below as S-Z.
Table 11
Experiment design for ruggedness studies (minor changes)
| Factor value F | Combination of determinations number | |||||||
|---|---|---|---|---|---|---|---|---|
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | |
| For calculations see examples for ruggedness testing in 3.3. | ||||||||
| A/a | A | A | A | A | a | a | a | a |
| B/b | B | B | b | b | B | B | b | b |
| C/c | C | c | C | c | C | c | C | c |
| D/d | D | D | d | d | d | d | D | D |
| E/e | E | e | E | e | e | E | e | E |
| F/f | F | f | f | F | F | f | f | F |
| G/g | G | g | g | G | g | G | G | g |
| Observed result R | S | T | U | V | W | X | Y | Z |