Laboratory precision

When two substances react, they react in exact amounts. You can determine what amounts of the two reactants are needed to react completely with each other by means of mole ratios based on the balanced chemical equation for the reaction. In the laboratory, precise amounts of the reactants are rarely used in a reaction. Usually, there is an excess of one of the reactants. As soon as the other reactant is used up, the reaction stops. The reactant that is used up is called the limiting reactant. Based on the quantities of each reactant and the balanced chemical equation, you can predict which substance in a reaction is the limiting reactant. 

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). 

Intralaboratory or within-laboratory precision refers to the precision of a test method when the results are obtained by the same operator in the same laboratory using the same apparatus. In some cases, the precision is applied to data gathered by a different operator in the same laboratory using the same apparatus. Thus, intralaboratory precision has an expanded meaning insofar as it can be applied to laboratory precision. 

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. 

Method precision refers to the variability in measurement of the same sample. There are three main components of method precision repeatability (also known as system or intraassay precision), intermediate precision (also known as inter-assay or intra-laboratory precision), and reproducibility precision (also known as ruggedness, overall or inter-laboratory... 

Precise High level of within-run, across-day, and across-laboratory precision assay must be reproducible... 

This simple function is most useful in setting acceptance criteria for analytical method inter-laboratory precision. Note that the concentration, C, is relative so that 100% has a value of 1, i.e. 10 . Table 19 lists values in decreasing powers of 10. 

In addition to the THM methods, EMSL-Cincinnati has developed purge and trap methods for selected halogenated (29) and aromatic (30) compounds that are considered to be chemical indicators of industrial contamination. The methods are applicable to 47 halogenated compounds (Method 502) and 33 compounds that have ionization potentials less than 10.2 eV and that are aromatic or contain a doubly bonded carbon (Method 503). Seven of these compounds are halogenated and are also included in the method for halogenated compounds. Another method, Method 524 (31), provides for GC-MS determination of 28 purgeable volatiles. Single laboratory precision and accuracy data for these compounds are provided in the EMSL methods. 

When conducting an inspection, several target areas must be evaluated. Control limits or "charts" are helpful and should be established by plotting the defined limits of acceptable quality control. These charts are important tools for assessing laboratory precision, accuracy, and reproducibility. They can be based on a curve established from the high, mid, and low concentrations of a standard analyte. Either the mid level or the average of the three concentrations then becomes the mid-line for the control chart. Acceptable levels of fluctuation for routine mid-level standards,... 

Analytical laboratories establish the accuracy of the performed methods through the preparation and analysis of laboratory control samples, the same samples that are used for the determination of laboratory precision. Based on a statistical evaluation of the recoveries from these interference-free matrices, laboratories derive the recovery acceptance criteria, called laboratory control limits. 

To determine laboratory precision as the RPD between the LCS and the LCSD recoveries... 

Rephcate samples should be collected at the same time (preferably a split of one sample rather than by collecting two or more concurrent samples in the field) and undergo the same filtration, preservation and storage. These replicate samples measure the variability of the processing techniques and the laboratory precision, but exclude field-sampling variability. 

The parameters describing the scattering of a test result under repeatability and reproducibility conditions are the corresponding standard deviations. The repeatability limit, "r , is the within-laboratory precision and describes the maximum expected value of the difference between two individual test results obtained under repeatabil-... 

As a result of the pre-validation work, which included a within-laboratory precision experiment carried out in two different laboratories at concentrations of 2.1 mg HMDA and 12.1 mg EDA per kg food simulant, the performance characteristics in Table 10-7 were obtained at the 95 % probability level. 

M. Thompson, Recent trends in inter-laboratory precision at ppb and sub-ppb concentrations in relation to fitness for purpose criteria in proficiency testing, Analyst, 125 (2000), 385-386. 

A Proficiency Test (PT) is defined as the study of laboratory performance by means of ongoing interlaboratory test comparisons . It is also known as an external quality assessment scheme, external laboratory performance check or external quality assurance (EQA). There are many such schemes run by independent external bodies for different analytes in a variety of matrices. Evidence in published papers shows that the performance of analytical laboratories improves as a result of participating in Proficiency Testing schemes and the betw een-laboratory precision can improve, sometimes dramatically. This is especially true in the early years of participation. 

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],... 

Much is made of detection limits in environmental analysis. Much of the modern concern about chemicals in the environment stems from the ability of the analytical chemist to analyse ever lower concentrations. Less attention is given to a limit of determination. Usually the RSD is the best possible for the given method, and because intra-laboratory precision, or even simple repeatability, is quoted, this is usually accepted. 

Ross JW, Lawson NS. Analytical goals, concentration relationships, and the state of the art for clinical laboratory precision. Arch Pathol Lab Med 1995 119 495-513. 

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. 

The poor precision of some hospital departments of clinical chemistry has been described in many national and international surveys. Lack of precision may lead to characterization of a normal result as abnormal. Such lack of precision may be due to lack of scientific ability on the part of those obtaining the result, but it must be emphasized that even in the best of laboratories such "drifts in precision do occur. A clear and true indication of laboratory precision is important in all patient investigations it is even more important when unsolicited information is being provided. 

Precision describes the scatter of measured values around the average of the measured values. Precision is much more important for biosensor performance than accuracy. Accuracy can be influenced by systematic errors that can be corrected. However, a sensor producing values that are scattered will not be regarded as very rehable. Some confusion can be found in the use of terms that are measures for the precision of a sensor, repeatability and reproducibility. Many papers report work as highly reproducible even though reproducibihty is defined as the hetween-laboratory precision. That simply means that a sensor s measurements are reproducible if the same results are obtained in different laboratories with the same sensor architecture [207]. This is, however, rarely tested. Most authors really test the repeatability of a sensor. Repeatabihty is the in-laboratory precision. In-laboratory precision means that the sensor yields the same value of, for example, concentration in repeated measurements. Repeatabihty also means that the same sensor architecture will yield the same result if manufactured in the same laboratory. It would hence make more sense to speak about the standard deviation of a single sensor and to analyze the repeatability of the respective sensor architecture than to speak about reproducibihty if the latter has not been tested. 

Performance of the method in one laboratory. The assessment of the precision achieved with the method in one laboratory, still compared to the a priori performance of the method a, but within an interlaboratory study is similar to the approach used above with the within-laboratory precision. With the consideration of the number p of sets of data delivered by the participants ... 

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

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