(sumbmitted to the Commission for endorsement)
PREAMBLE
The purpose of this document is to provide harmonized analytical terminology specifically related to the work of the Codex Committee on Methods of Analysis and Sampling. It was recognized that various disciplines, e.g., analytical chemistry and metrology, use the same terms in different ways and this causes confusion. Even within the chemical field the meaning of the same term may vary among commodity areas. Therefore, the Codex Committee on Methods of Analysis and Sampling/Codex Alimentarius Commission needs to harmonize core terminology with clear definitions to ensure that analysts mean the same thing when using a particular term.
VOCABULARY[35]
RESULT: The final value reported for a measured or computed quantity, after performing a measuring procedure including all subprocedures and evaluations. {IUPAC-1994}
NOTES: {VIM}
1 When a result is given, it should be made clear whether it refers to:
- the indication [signal]2 A complete statement of the result of a measurement includes information about the uncertainty of measurement.
- the uncorrected result
- the corrected result
and whether several values were averaged.
ACCURACY (AS A CONCEPT): The closeness of agreement between the reported result and the accepted reference value.
NOTE:
The term accuracy, when applied to a set of test results, involves a combination of random components and a common systematic error or bias component. {ISO 3534-1} When the systematic error component must be arrived at by a process that includes random error, the random error component is increased by propagation of error considerations and is reduced by replication.ACCURACY (AS A STATISTIC): The closeness of agreement between a reported result and the accepted reference value. {ISO 3534-1}
NOTE:
SAccuracy as a statistic applies to the single reported final test result; accuracy as a concept applies to single, replicate, or averaged values.TRUENESS: The closeness of agreement between the average value obtained from a series of test results and an accepted reference value.
NOTES:
1 The measure of trueness is usually expressed in terms of bias. {ISO 3534-1}BIAS: The difference between the expectation of the test results and an accepted reference value. {ISO 3534-1}
2 Trueness has been referred to as "accuracy of the mean."
NOTES:
1 Bias is the total systematic error as contrasted to random error. There may be one or more systematic error components contributing to bias. A larger systematic difference from the accepted reference value is reflected by a larger bias value. {ISO 3534-1}PRECISION: The closeness of agreement between independent test results obtained under stipulated conditions. {ISO 3534-1}2 When the systematic error component(s) must be arrived at by a process that includes random error, the random error component is increased by propagation of error considerations and reduced by replication.
NOTES: {ISO 3534-1}
1 Precision depends only on the distribution of random errors and does not relate to the true value or to the specified value.Repeatability [Reproducibility]: Precision under repeatability [reproducibility] conditions. {ISO 3534-1}2 The measure of precision is usually expressed in terms of imprecision and computed as a standard deviation of the test results. Less precision is reflected by a larger standard deviation.
3 "Independent test results" means results obtained in a manner not influenced by any previous result on the same or similar test object. Quantitative measures of precision depend critically on the stipulated conditions. Repeatability and reproducibility conditions are particular sets of extreme conditions.
Repeatability conditions: Conditions where independent test results are obtained with the same method on identical test items in the same laboratory by the same operator using the same equipment within short intervals of time. {ISO 3534-1}
Reproducibility conditions: Conditions where test results are obtained with the same method on identical test items in different laboratories with different operators using different equipment. {ISO 3534-1}
NOTE:
When different methods give test results that do not differ significantly, or when different methods are permitted by the design of the experiment, as in a proficiency study or a material-certification study for the establishment of a consensus value of a reference material, the term "reproducibility" may be applied to the resulting parameters. The conditions must be explicitly stated.Repeatability [Reproducibility] standard deviation: The standard deviation of test results obtained under repeatability [reproducibility] conditions. {ISO 3534-1}
NOTES: {ISO 3534-1}
1 It is a measure of the dispersion of the distribution of test results under repeatability [reproducibility] conditions.Repeatability [Reproducibility] limit: The value less than or equal to which the absolute difference between two test results obtained under repeatability [reproducibility] conditions may be expected to be with a probability of 95%. {ISO 3534-1}2 Similarly "repeatability [reproducibility] variance" and "repeatability [reproducibility] coefficient of variation" could be defined and used as measures of the dispersion of test results under repeatability [reproducibility] conditions.
NOTES:
1 The symbol used is r [R]. {ISO 3534-1}APPLICABILITY: The analytes, matrices, and concentrations for which a method of analysis may be used satisfactorily to determine compliance with a Codex standard.2 When examining two single test results obtained under repeatability [reproducibility] conditions, the comparison should be made with the repeatability [reproducibility] limit
r [R] = 2.8 sr[sR]. {ISO 5725-6, 4.1.4}3 When groups of measurements are used as the basis for the calculation of the repeatability [reproducibility] limits (now called the critical difference), more complicated formulae are required that are given in ISO 5725-6:1994, 4.2.1 and 4.2.2.
NOTE:
In addition to a statement of the range of capability of satisfactory performance for each factor, the statement of applicability (scope) may also include warnings as to known interference by other analytes, or inapplicability to certain matrices and situations.SPECIFICITY: The property of a method to respond exclusively to the characteristic or analyte defined in the Codex standard.
NOTES:
1 Specificity may be achieved by many means: It may be inherent in the molecule (e.g., infrared or mass spectrometric identification techniques), or attained by separations (e.g., chromatography), mathematically (e.g., simultaneous equations), or biochemically (e.g., enzyme reactions). Very frequently methods rely on the absence of interferences to achieve specificity (e.g., determination of chloride in the absence of bromide and iodide).SENSITIVITY: Change in the response divided by the corresponding change in the concentration of a standard (calibration) curve; i.e., the slope, si, of the analytical calibration curve.2 In some cases specificity is not desired (e.g., total fat, fatty acids, crude protein, dietary fibre, reducing sugars).
NOTE:
This term has been used for several other analytical applications, often referring to capability of detection, to the concentration giving 1% absorption in atomic absorption spectroscopy, and to ratio of found positives to known, true positives in immunological and microbiological tests. Such applications to analytical chemistry should be discouraged.NOTES: {IUPAC-1987}
1 A method is said to be sensitive if a small change in concentration, c, or quantity, q, causes a large change in the measure, x; that is, when the derivative dx/dc or dx/dq is large.RUGGEDNESS: The ability of a chemical measurement process to resist changes in results when subjected to minor changes in environmental and procedural variables, laboratories, personnel, etc. {IUPAC-1995}2 Although the signal may vary with the magnitude of ci or qi, the slope, si, is usually constant over a reasonable range of concentrations. si may also be a function of the c or q of other analytes present in the sample.
· The following set of definitions are from {IUPAC-1994-2}, with some minor revisions for clarity:
INTERLABORATORY STUDY: A study in which several laboratories measure a quantity in one or more "identical" portions of homogeneous, stable materials under documented conditions, the results of which are compiled into a single document.
NOTE:
The larger the number of participating laboratories , the greater the confidence that can be placed in the resulting estimates of the statistical parameters. The IUPAC-1987 protocol (Pure & Appl. Chem., 66, 1903-1911(1994)) requires a minimum of eight laboratories for method-performance studies.Method-Performance Study: An interlaboratory study in which all laboratories follow the same written protocol and use the same test method to measure a quantity in sets of identical test samples. The reported results are used to estimate the performance characteristics of the method. Usually these characteristics are within-laboratory and among-laboratories precision, and when necessary and possible, other pertinent characteristics such as systematic error, recovery, internal quality control parameters, sensitivity, limit of determination, and applicability.
NOTES:
1 The materials used in such a study of analytical quantities are usually representative of materials to be analyzed in actual practice with respect to matrices, amount of test component (concentration), and interfering components and effects. Usually the analyst is not aware of the actual composition of the test samples but is aware of the matrix.Laboratory-Performance (Proficiency) Study: An interlaboratory study that consists of one or more measurements by a group of laboratories on one or more homogeneous, stable, test samples by the method selected or used by each laboratory. The reported results are compared with those from other laboratories or with the known or assigned reference value, usually with the objective of improving laboratory performance.2 The number of laboratories, number of test samples, number of determinations, and other details of the study are specified in the study protocol. Part of the study protocol is the procedure which provides the written directions for performing the analysis.
3 The main distinguishing feature of this type of study is the necessity to follow the same written protocol and test method exactly.
4 Several methods may be compared using the same test materials. If all laboratories use the same set of directions for each method and if the statistical analysis is conducted separately for each method, the study is a set of method-performance studies. Such a study may also be designated as a method-comparison study.
NOTES:
1 Laboratory-performance studies can be used to support accreditation of laboratories or to audit performance. If a study is conducted by an organization with some type of management control over the participating laboratories - organizational, accreditation, regulatory, or contractual - the method may be specified or the selection may be limited to a list of approved or equivalent methods. In such situations, a single test sample is insufficient to judge performance. It is expected that the results from 1 of every 20 tests will be outside the value for the calculated mean ± twice the standard deviation, due solely to random fluctuations.Material-Certification Study: An interlaboratory study that assigns a reference value ("true value") to a quantity (concentration or property) in the test material, usually with a stated uncertainty.2 Sometimes a laboratory-performance study may be used to select a method of analysis that will be used in a method-performance study. If all laboratories, or a sufficiently large subgroup, of laboratories , use the same method, the study may also be interpreted as a method-performance study, provided that the test samples cover the range of concentration of the analyte.
3 Separate laboratories of a single organization with independent facilities, instruments, and calibration materials, are treated as different laboratories.
NOTE:
A material-certification study often utilizes selected reference laboratories to analyze a candidate reference material by a method(s) judged most likely to provide the least-biased estimates of concentration (or of a characteristic property) and the smallest associated uncertainty.SOURCE MATERIALS
(1) VIM: "International Vocabulary of Basic and General Terms in Metrology" Second edition, 1993, International Organization for Standardization, Geneva, Switzerland, 58 pp.(2) ISO 3534-1:1993: Statistics -Vocabulary and Symbols -Part 1: Probability and general statistical terms. International Organization for Standardization, Geneva, Switzerland, 47 pp.
ISO 3534-2:1993: Statistics -Vocabulary and Symbols - Part 2: Statistical quality control. International Organization for Standardization, Geneva, Switzerland, 33 pp.
(3) ISO 5725-6:1994: Accuracy (trueness and precision) of measurement methods and results - Part 6: Use in practice of accuracy values. International Organization for Standardization, Geneva, Switzerland, 41 pp.
(4) IUPAC-1987: Freiser, H. and Nancollas, G.H. "Compendium of Analytical Nomenclature. Definitive Rules 1987." Second Edition, 1987. Blackwell Scientific Publications, Oxford UK, pp.
(5) IUPAC-1994: Currie L.A. and Svehla, G. "Nomenclature for the Presentation of Results of Chemical Analysis. (IUPAC Recommendations 1994)." Pure & Appl. Chem. 66, 595-608 (1994).
(6) IUPAC-1994-2: Horwitz, W. "Nomenclature of Interlaboratory Analytical Studies (IUPAC Recommendations 1994)" Pure & Appl. Chem. 66, 1903-1911 (1994).
(7) IUPAC-1995: Currie L.A. "Nomenclature in Evaluation of Analytical Methods Including Detection and Quantitation Capabilities. (IUPAC Recommendations 1995)." Pure & Appl. Chem. 67, 1699-1723(1994).
