Accuracy and precision of microbiological tests

In this section the terms accuracy, systematic error and precision are defined as proposed by Havelaar et al. [4], These concepts are derived from quantitative analytical chemistry but can just as well be used in quantitative microbiology. According to ISO/DIS 6107-8 [33], accuracy is defined as the degree of similarity between the measurement and the true value of the measured quantity. In a microbiological context the true value is the number of colony-forming particles in the sample. The accuracy is related to the overall error of measurement, that is the difference between the measurement and the true value of the measured quantity. The overall error can be divided into three different components  

Firstly, there is the. systematic deviation between measurement and true value that cannot be avoided with the characteristics of the sample being analysed. Secondly, there is a deviation that is typical for a laboratory arising from the peculiarities of that laboratory. Thirdly, there is a random error that is made at a particular instant that would be different when the laboratory repeated its measurement. In summary, the model for the result of a single measurement is  

In section 2.3.3.1 the existence of methods giving a presence/absence result are mentioned. With the results from such tests a breakdown of the overall error becomes quite complicated. Only two types of error can be clearly discerned, either the method incorrectly identifies a sample as positive (false positive) or it incorrectly identifies a sample as negative (false negative). A method is fully accurate if it avoids both types of error. 

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Quality in chemical and biological analysis 23.3.2. Accuracy and precision of microbiological tests... 

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