Precision intralaboratory

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. 

Figure 4.22. Pareto chart of the contributions to the uncertainty of the quantitative NMR analysis of Profenofos. The effects are a (intra), the intralaboratory precision P(std), the purity of the proton standard w, weighings of unknown and standard MW, the molecular weights of unknown and standard. (Data kindly supplied by T Saed Al-Deen.)...

The repeatability (sr) can be used to check duplicate repeats during normal operation of a method (see chapter 1). On its own, repeatability is not a complete basis for estimation of measurement uncertainty because it omits many effects that contribute to the bias of measurements made within a single laboratory. However, combined with a good estimate of the run bias, the intralaboratory precision, obtained from quality control data, can be used to give an estimate of measurement uncertainty. See section 6.6.3.2 for details on correction for bias and recovery. 

Repeatability (intralaboratory precision) is the degree of agreement between independent test results produced by the same analyst using the same test method and equipment on random aliquots of the same sample within a short time period (EPA, 1998a). Repeatability is calculated as the RPD for two measurements, as the variance or the standard deviation for more than two measurements. 

Each analyst performing analysis must be trained in the procedure, and the training must be documented in the laboratory records. The training establishes single analyst precision through the preparation and analysis of a blank and four LCSs. (Single analyst precision is the repeatability or intralaboratory precision, a secondary DQI.)... 

Reproducibility is defined as the long-term variability of the measurement process, which may be determined for a method run, within a single laboratory, but on different days. Reproducibility also applies to a method, either run by different operators, different instruments, or a combination of the above. The reproducibility standard deviation is typically twofold to threefold larger than that for repeatability. Precision is often expressed relative to 1 day as intraday (within-day) precision or relative to a period of days, as interday (between days) precision. Reproducibility, in the sense of intralaboratory precision, is related to the procedure being performed at two or more laboratories as in, e.g., a collaborative study. 

Intralaboratory precision studies are classified as intraassay, where the profile is obtained doing the replication in only one run, using the same batch of reagents, or interassay, where the precision profile is obtained by comparison of runs done on different days. Poorer precision is generally obtained in interassay studies. 

Ehrmeyer SS> Laessig RH. A computer model to translate federal proficiency testing performance standards for pH/blood gases into intralaboratory precision and accuracy requirements. In Methodology and clinical applications of blood gases, pH, electrolytes and sensor technology. 

Reproducibility (ICH) Precision of repeated measurements between analytical laboratories also termed intralaboratory precision. 

In an interlaboratory study with nine participating laboratories a TSP-LC-MS method was evaluated in order to determine diuron and linuron. Depending on the compounds, the intralaboratory precisions range from 6.5-33 with a relative standard deviation (RSD) of 1% [254]. After linuron had been submitted to photochemical degradation, TSP-LC-MS and MS/MS was used to characterise the degradation products. A tentative photodegradation pathway for the different pesticides in water was postulated by Durand et al. [256]. 

In this example, the intralaboratory precision is the greatest contributor to the uncertainty. There would be nothing gained, for example, in performing the mass measurements on a more precise balance. 

At present, there is no completely valid method available to directly calculate the interlaboratory precision from the intralaboratory precision or vice versa. This is unfortunate because frequently only one type of precision estimate is available for a method. ITowever, in general, the following comments are applicable. First, the intralaboratory precision should be smaller than the interlaboratory precision because of additional variables in the latter. Second, if the interlaboratory precision is much larger than the intralaboratory precision, this indicates that the method is very technique-sensitive. Such information can be very important when considering a project that might involve several laboratories analyzing the same sample. 

A semi-empirical formula that is useful in relating the intralaboratory precision with the interlaboratory precision is represented by the following relationship ... 

However, the relative deviations for interlaboratory reproducibility sr are considerable, i.e. 26.6% (AO-1) and 12.3% (AO-2). These results (Table 6.23) show that the determination of antioxidants in polyolefins is not a trivial matter. For cases where the interlaboratory precision is much larger than the intralaboratory precision there is obviously lack of robustness of the analytical methods used. 

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