Precision between laboratory

Method performance study All laboratories follow the same written protocol and use the same test method to measure a quantity (usually concentration of an analyte) in sets of identical test samples. The results are used to estimate the performance characteristics of the method, which are usually within-laboratory- and between-laboratory precision and - if relevant - additional parameters such as sensitivity, limit of detection, recovery, and internal quality control parameters (IUPAC Orange Book [1997, 2000]). 

We will address aspects of reproducibility, which has previously been defined as, the precision between laboratories . It has also been defined as total between-laboratory precision . This is a measure of the ability of different laboratories to evaluate each other. Reproducibility includes all the measurement errors or variances, including the within-laboratory error. Other terms include precision, defined as the closeness of agreement between independent test results obtained under stipulated conditions [3] and repeatability, or the precision for the same analyst within the same laboratory, or within-laboratory precision . Note that for none of these definitions do we require the true value for an analytical sample . In practice we do not know the true analyte value unless we have created the sample, and then it is only known to a given certainty (i.e., within a determined uncertainty). 

Interlaboratory or between-laboratory precision is defined in terms of the variability between test results obtained on the aliquots of the same homogeneous material in different laboratories using the same test method. 

Comparing these collaborative study results to the average expected from the Horwitz curve, a composite RSD, curve of more than 300 collaborative studies. For SFB the average Horwitz ratios were 0.8, 0.59 and 0.55 for the three spiking levels for PCD the ratios were 1.38, 0.33 and 0.96. Both SFB and PCD showed acceptable within and between laboratory precision. (The Horwitz ratio compares the RSD, at the various levels and in the various matrices of this method with those RSD, values predicted based historically on methods for a wide variety of analytes reported in AOAC collaborative studies a ratio <2 is considered to have acceptable and typical precision (12). 

The precision of an analytical method is the closeness of a series of individual measurements of an analyte when the analytical procedure is applied repeatedly to multiple aliquots of a single homogeneous volume of biological matrix [16], The precision is calculated as coefficient of variation (C.V.), i.e., relative standard deviation (RSD). The measured RSD can be subdivided into three categories repeatability (intra-day precision), intermediate precision (inter-day precision) and reproducibility (between laboratories precision) [16, 78, 79, 81],... 

Available analytical performance data for fecal fat measurements in the UK also indicate that the test should now be consigned to history. Eighty-two per cent of laboratories use no internal quality control and EQA is impractical. When the titration step was assessed in an EQA exercise, between-laboratory coefficients of variation for three samples ranged from 31% to 42%. Infrared spectroscopy offers the possibility of improved within- and between-laboratory precision for fecal fat measurements, but does not address the problems of dietary input and sample collection, and is unlilcely to be available to most laboratories. 

Between-laboratory precision see Interlaboratory precision, Intermediate precision, and Precision). 

Method precision results, as achieved in a laboratory developing a method, will often be superior to the variability achieved by another laboratory that may later use the method. If a method cannot achieve a suitable standard of performance in the laboratory where it was developed, it cannot be generally expected to do any better in other laboratories. Typically, the intermediate precision determined within a laboratory is 1-2% greater than the precision as determined for a single analyst, while the between-laboratory precision, or reproducibility, is again 1 -2% higher than the within-laboratory precision. 

For analytical procedures, precision may be specified as either intralaboratory (within-laboratory) or interlaboratory (between-laboratory) precision. Estimates... 

The underlying principles of the bootstrap approach are easily understood, and the iterative calculations are simple, for example as a macro written for Minitab (see Bibliography). The most important applications in analytical practice are likely to be more complex situations than the one in the above example. Suggested uses have included the estimation of between-laboratory precision in collaborative trials (see Chapter 4), and in the determination of the best model to use in multivariate calibration (see Chapter 8). 

The variability inherent in any measurement of concrete properties should be low enough to give acceptable levels of both within-batch and between-laboratory precision. 

Because this separation is not subject to precise standards today, the resulting wide variations make comparisons between laboratories risky. 

Precision is a measure of the spread of data about a central value and may be expressed as the range, the standard deviation, or the variance. Precision is commonly divided into two categories repeatability and reproducibility. Repeatability is the precision obtained when all measurements are made by the same analyst during a single period of laboratory work, using the same solutions and equipment. Reproducibility, on the other hand, is the precision obtained under any other set of conditions, including that between analysts, or between laboratory sessions for a single analyst. Since reproducibility includes additional sources of variability, the reproducibility of an analysis can be no better than its repeatability. 

Measurements of individual laboratory performance provides for comparisons between laboratories. It then follows to ask why some laboratories report data that are more accurate and precise than do their peers, and a well designed external quality assessment scheme allows investigation of some of the important factors (see below). A comparison of performance between individual laboratories also helps to stimulate those who are not so successful to improve (or abandon the assay) and those who do well to continue with their expertise. Finally, changes of performance may be monitored as a consequence of some new factor, e.g. purchase of a new piece of equipment, work carried out by a different analyst, change to the methodology etc. 

The precision of an analytical method is a measure of the variability of repetitive measurements. Contributions from numerous sources affect precision, but the major components are within-laboratory (repeatability) and between-laboratory (reproducibility) variations. Precision is expressed as the relative standard deviation (or CV)... 

The precision of recovery is determined under repeatability and reproducibility conditions. The more important between-laboratory reproducibility is calculated as relative standard deviation (RSDr) and compared with the RSDr, which is estimated from the Horwitz equation using the same analyte concentration. For good methods this ratio should be about 1, but a method will usually be accepted if the ratio is not larger than 2. 

Verification implies that the laboratory investigates trueness and precision in particular. Elements which should be included in a full validation of an analytical method are specificity, calibration curve, precision between laboratories and/or precision within laboratories, trueness, measuring range, LOD, LOQ, robustness and sensitivity. The numbers of analyses required by the NMKL standard and the criteria for the adoption of quantitative methods are summarized in Table 10. 

In both cases, the validation process could be divided into two components reliability and relevance. Reliability (precision) is the ability of the method to obtain reproducible results between laboratories while relevance (accuracy) is defined by the comparison of the output from the applied method with the ones obtained using a gold standard test which works as a reference at international levels. 

Bias is allowed between laboratories when constant and deterministic. For any method of optimization we must consider the requirements for precision and bias, specificity, and MDL. 

The uncertainty of a measurement is always a very important consideration but it is especially so with durability tests. In addition to the basic physical test methods, there are the complications of exposure conditions, tests spanning long times and the process of extrapolating results to make predictions. The combination of these factors will inevitably lead to large uncertainties that can very easily be of such a magnitude that any conclusions are meaningless. Terms used to describe precision include repeatability, which refers to within laboratory variation and reproducibility, which refers to variation between laboratories. 

Reproducibility (ruggedness) Precision between laboratories, usually determined by collaborative studies ... 

Reproducibility represents the precision of the method between two or more laboratories and it is typically assessed during method transfer between laboratories, but may be assessed during method validation when more than one laboratory will be performing the method. Reproducibility would also be reported as the SD or RSD value of the mean results between laboratories. These data are not part of the marketing authorization application. 

Reproducibility Reproducibility measures the precision between laboratories. This parameter is considered in the standardization of an analytical procedure (e.g., inclusion of procedures in pharmacopeias and method transfer between different laboratories). 

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