Precision of a method

Precision and bias both influence a result - this is illustrated in Figure 4.4 and discussed below. As mentioned in Section 4.3.2, the precision of a method is a statement of the closeness of agreement between independent test results obtained... 

The estimation of the overall precision of a methodfrom its unit operations A frequent problem in analysis is the estimation of the overall precision of a method before it has been used or when insufficient data are available to carry out a statistical analysis. In these circumstances the known precision limits for the unit operations of the method (volume measurement, weighing, etc.) may be used to indicate its precision. Table 2.6 gives the normal precision limits for Grade A volumetric equipment. 

As seen above, all official methods of analysis are required to include precision data. These may be obtained by subjecting the method to a collaborative trial conforming to an internationally agreed protocol. A collaborative trial is a procedure whereby the precision of a method of analysis may be assessed and quantified. The precision of a method is usually expressed in terms of repeatability and reproducibility values. Accuracy is not the objective. 

Precision is a measure of the ability of the method to generate reproducible results. The precision of a method is evaluated using three separate determinations for repeatability, intermediate precision, and reproducibility. 

The study of the precision of a method is often the most time and resource consuming part of a method validation program, particularly for methods that are developed for multiple users. The precision is a measure of the random bias of the method. It has contributions fi om the repeatability of various steps in the analytical method, such as sample preparation and sample injection for HPLC [5-9], and from reproducibility of the whole analytical method fiom analyst to analyst, fiom instrument to instrument and fiom laboratory to laboratory. As a reproducibility study requires a large commitment of time and resources it is reasonable to ensure the overall ruggedness of the method before it is embarked upon. 

The precision of a method is the extent to which the individual test results of multiple injections of a series of standards agree. The measured standard deviation can be subdivided into three categories repeatability, intermediate preci-... 

Random error (commonly referred to as noise) produces results that are spread about the average value. The greater the degree of randomness, the larger the spread. Statistics are often used to describe random errors. Random errors are typically ones that we have no control over, such as electrical noise in a transducer. These errors affect the precision or reproducibility of the experimental results. The goal is to have small random errors that lead to good precision in our measurements. The precision of a method is determined from replicate measurements taken at a similar time. 

A relevant juristical statement about the precision of a method can only be made after defining the performance characteristics obtained from a round robin or interlaboratory trial study, as for instance described in ISO 5725 (ISO 1994). This study is used to determine the statistical key data about the precision of a method. The international standard ISO 5725 has been adopted by many countries. 

The precision has been defined in the ISO 5725-1 standard as the closeness of agreement between independent test results obtained under stipulated conditions [28]. Many factors may contribute to the variability of test results obtained with the same method on identical samples, including (but not limited to) the operators), the equipment, the reagents, the RM(s), the environment, the time between measurements, and the laboratories. The maximal variability of test results is explained by the reproducibility (R) of the method. All factors that have influence on the variability of a test method should be taken into consideration when assessing the reproducibility. The repeatability (r) is assessed by keeping all the above-mentioned factors constant (e.g., same operator, same equipment, same laboratory, short time interval). It is a measure of the minimum variability of a method. The intermediate precision is situated between the two extreme measures of precision repeatability and reproducibility. The terms within-laboratory reproducibility (w), long-term precision, and so on, are often used to demonstrate the intermediate precision of a method. For a correct interpretation of the intermediate precision, the factors that have been taken into account should be known. 

Precision. The precision of a method is the degree of scatter of the results and is usually reported as a percentage relative standard deviation. It is often subdivided into repeatability (precision on replicate measurements of the same solution) and reproducibility (precision of the results from measurements of different solutions, i.e. of the complete method). 

Reproducibility, or precision, of a method relates to how individual estimates fluctuate around the average value. The magnitude of the fluctuation in the population is expressed by the parameter variance Variance is the average of the squared deviations about p for all values x, in the population E(x, f/N. An unbiased estimate of is obtained from the deviation of each value (x,) around the mean (x ) for a sample taken from the population = S(x, — x) /... 

As the matrix effect is (strongly) compound dependent, an ANIS may be differently affected by the matrix than the analyte. The precision of a method can be significantly improved by modifying the mobile-phase conditions in such a way that analyte and ANIS co-elute [87]. An improvement in the intra-day precision from 5.2-16.2%RSD with separation between 2-C-ethynylcytidine as analyte and 3 -C-ethylcitidine as ANIS to 2.7-4.2%RSD without separation was achieved in this way. 

The precision of a method is the extent of agreement among individual test results, when the procedure is repeatedly applied to multiple samplings. Precision can be divided into three categories 1) Repeatability 2) intermediate precision and 3) reproducibility. 

However, one important point should be kept in mind when statistically testing the model fit The higher the precision of a method, the higher the probability to detect a statistically significant deviation from the assumed calibration model [1, 6, 9]. Therefore, the practical relevance of the deviation from the assumed model should also be taken into account. If the accuracy data (bias and precision) are within the required acceptance limits or an alternative calibration model is not applicable, slight deviations from the assumed model may be neglected [6, 9],... 

The assessment of the precision of a method before its introduction into routine use requires a measurement of its repeatability, but the reproducibility cannot be determined until the method has itself been in routine operation for a sufBcient period of time (see Section 2 for definitions). Most published descriptions of techniques provide data about their repeatability when referring to precision, but there are some published figures specifically for reproducibility (e.g.. Cl, S7, T2). Having decided that a method is sufficiently reliable (on the basis of the SD for repeatability) to be introduced into routine practice, the SD for its reproducibility should be obtained as soon as possible, for instance by calculating the SD for the results of analyses on control sera included in the first 20 batches of determinations this SD reflects the between-batch variation. Subsequent monitoring of the method should then be related to this first value for reproducibility, if it compares favorably with the SD for repeatability. 

Accuracy and precision are the most important characteristics of an analytical method they give the best indication of random and systematic error associated with the analytical measurement. Systematic error refers to the deviation of an analytical result from the true value, and therefore affects the accuracy of a method. One the other hand, random errors influence the precision of a method (Kallner et al., 1999). Ideally, accuracy and precision should be assessed at multiple concentrations within the linear range of the assay (low, medium and high concentration). 

The precision of a method can also be assessed through an interlaboratory study. This gives a much better estimate of the real precision the method and the laboratory can achieve. The way to properly organise such studies is described in Chapter 12. 

Definition The precision of an analytical method is the extent of the agreement between the measured values in a series of analyses of a homogeneous sample. The precision of a method is normally given by the standard deviation (.S ) or relative standard deviation (coefficient of variation or. Srei). The precision is a measure of the variabUity of the analytical method under normal laboratory conditions. 

Sensitivity is closely related to the specificity and precision of a method. As an extremely important aspect of any method for plasma steroids, and of many for urinary steroids, it has been much discussed. In terms of the best levels of signal-to-noise ratio obtained with pure substances under ideal conditions, the mass spectrometer attached directly to a good GLC column is probably the most sensitive analytical method currently available. In terms of steroids in natural sources the most sensitive methods at present established are those using the isotopic-displacement (protein-binding) technique (0.1-0.5 ng) and electron-... 

The repeatability standard deviation is defined as The precision of a method expressed as the standard deviation of independent determinations performed by a single analyst using the same apparatus and techniques. Hence the repeatability is what governs any replicate measurements made in your laboratory by you on the test material. The reference to independent measurements means that a number of separate test portions should be weighed, dissolved, and measured, not that the same solution should be presented to the... 

Reproducibility standard deviation is The precision of a method expressed as the standard deviation of determinations performed in different laboratories. Remembering the discussion of chapter 1 we would expect the reproducibility to be greater than the repeatability, as each laboratory will have its own repeatability (which might be expected to be about the same) but the differences between the laboratories reflecting different biases will now add to this to give the reproducibility. Experience has shown that the interlaboratory reproducibility is about two to three times the repeatability. 

The pooled standard deviation, defined below, is sometimes used to obtain an im-. proved estimate of the precision of a method, and it is used for calculating the pre-cision of the fwo sets of data in a paired t test. JThat is, rather than relying on a "Impe set of data to describe the precision of a method, it is sometimes preferable to perform several sets of analyses, for example, on different days, or on Afferent samples with slightly different compositions. If the indeterminate (random) enror is assumed to be the same jfor each set then the data of the different sets can be pooled. This provides a. more reliable estimate of the precision of a method than is obtained from a single set. The pooled standard deviation Sp is given by... 

We have defined the precision of a method. Repeatability is the long-term precision over several weeks. Ruggedness refers to the precision of one lab over multiple days. 

The final goals of pesticide analyses are to obtain the cleanest possible samples, to determine the minimum possible concentration with the lowest limits of detection, and to avoid pesticide degradation during transfer to the laboratory. All this means that the accuracy and precision of a method for pesticide analysis will be directly dependent on the sample preparation procedure used. This operation is the most time-consuming and labor-intensive task in the analytical scheme. In response to the need for effective, robust, reliable sample preparation, a number of procedures have developed for fast, simple, and, if possible, solvent-free or solvent-minimized operation. Most such procedures, both conventional and new, are used for the analysis of pollutants in air, water, soils, sediments, and biota. ... 

Precision, which quantifies the variation between replicated measurements on test portions from the same sample material, is also an important consideration in determining when a residue in a sample should be considered to exceed a MRL or other regulatory action limit. Precision of a method is usually expressed in terms of the within-laboratory variation (repeatability) and the between-laboratory variability (reproducibility) when the method has been subjected to a multi-laboratory trial. For a single-laboratory method validation, precision should be determined from experiments conducted on different days, using a minimum of six different tissue pools, different reagent batches, preferably different equipment, and so on, and preferably by different analysts Repeatability of results when determined within a single laboratory but based on results from multiple analysts is termed intermediate precision Precision of a method is usually expressed as the standard deviation. Another useful term is relative standard deviation, or coefficient of variation (the standard deviation divided by the absolute value of the arithmetic mean result, multiplied by 100 and expressed as a percentage). 

The same group [66] studied the precision of a method based on the double-coating buffer kit over a 20-day period. They found out that the method is highly reproducible both in migration times and peak areas of Tf isoforms. Martello et al. [198] ran an interlaboratory comparison of a method based on the dynamic double coating of the capillary wall performed with commercially available reagents. Results from both laboratories showed high correlation. 

Precision The precision of a method is the degree of mutual agreement among a series of measurements from multiple sampling of the sample. The method should be validated for short-term repeatability, day-to-day reproducibility, instrument-to-instrument reproducibility, and laboratory-to-laboratory reproducibility. The precision is expressed as variance, standard deviation, or coefficient of variance. 

Precision The degree of mutual agreement between repeated individual measurements made on the same sample. The precision of a method may be excellent but its accuracy may be very poor. (See Fig. 5.1). The terms accuracy and precision are often misused and frequently interchanged. These definitions have been taken from recommended ASTM standards [1]. 

Used as part of a certification scheme, although this does not ensure traceability Determines the reproducibility precision of a method and if a CRM is used, the method or laboratory bias Similar to a collaborative trial but with a specific aim... 

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