Abstract
Reports act as an important feedback tool in External Quality Assessment (EQA). Their main role is to score laboratories for their performance in an EQA round. The most common scores that apply to quantitative data are Q- and Z-scores. To calculate these scores, EQA providers need to have an assigned value and standard deviation for the sample. Both assigned values and standard deviations can be derived chemically or statistically. When derived statistically, different anomalies against the normal distribution of the data have to be handled. Various procedures for evaluating laboratories are able to handle these anomalies. Formal tests and graphical representation techniques are discussed and suggestions are given to help choosing between the different evaluations techniques. In order to obtain reliable estimates for calculating performance scores, a satisfactory number of data is needed. There is no general agreement about the minimal number that is needed. A solution for very small numbers is proposed by changing the limits of evaluation. Apart from analyte- and sample-specific laboratory evaluation, supplementary information can be obtained by combining results for different analytes and samples. Various techniques are overviewed. It is shown that combining results leads to supplementary information, not only for quantitative, but also for qualitative and semi-quantitative analytes.
Highlights
Reports created by External Quality Assessment (EQA) providers serve as a major feedback tool towards the participating laboratories
If the distribution of the data reported by well performing laboratories approaches a normal distribution, Z-scores follow a standard normal distribution and the percentage of Z-scores that are beyond extreme values can be calculated exactly: 4.6% and 0.27% of the Z-scores will have an absolute value greater than 2 and 3, respectively
Evaluation methods applied for data gathered in EQA rounds vary widely, for continuous data, and for semi-quantitative and qualitative data
Summary
Reports created by External Quality Assessment (EQA) providers serve as a major feedback tool towards the participating laboratories. In addition to individual reports for each participant, summary reports containing general and anonymized information on method performance, variability and bias for di erent analytes could be included at the end of each round. Because of large di erences in EQA scheme design, evaluation procedures vary widely and depend on, among others, choices made for determining the assigned value, commutability of control samples or the way in which laboratories report their results in routine. Laboratories are marked for an out of consensus result if they report a value that is too far from the assigned value and prior to any interpretation, the EQA provider must determine the assigned value and a range of acceptable values around it [1,8,11,13]. The approach includes requirements derived from speci c studies or general studies like biological variability, and in a second instance, state of the art performance criteria as well
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