1.1 This practice establishes a uniform standard for computing the within-laboratory quantitation estimate associated with Z % relative standard deviation (referred to herein as WQE_Z %), and provides guidance concerning the appropriate use and application.

1.2 WQE_Z % is computed to be the lowest concentration for which a single measurement from the laboratory will have an estimated Z % relative standard deviation (Z % RSD, based on within-laboratory standard deviation), where Z is typically an integer multiple of 10, such as 10, 20, or 30. Z can be less than 10 but not more than 30. The WQE_{10 %} is consistent with the quantitation approaches of Currie (1)2 and Oppenheimer, et al. (2).

1.3 The fundamental assumption of the WQE is that the media tested, the concentrations tested, and the protocol followed in developing the study data provide a representative and fair evaluation of the scope and applicability of the test method, as written. Properly applied, the WQE procedure ensures that the WQE value has the following properties:

1.4 The WQE applies to measurement methods for which instrument calibration error is minor relative to other sources, because this practice does not model or account for instrument calibration error, as is true of most quantitation estimates in general. Therefore, the WQE procedure is appropriate when the dominant source of variation is not instrument calibration, but is perhaps one or more of the following:

1.5 Data Quality Objectives—For a given method, one typically would compute the WQE for the lowest RSD for which the data set produces a reliable estimate. Thus, if possible, WQE_10 % would be computed. If the data indicated that the method was too noisy, so that WQE_10 % could not be computed reliably, one might have to compute instead WQE_20 %, or possibly WQE_30 %. In any case, a WQE with a higher RSD level (such as WQE_50 %) would not be considered, though a WQE with RSD < 10 % (such as WQE_5 %) could be acceptable. The appropriate level of  RSD is based on the data quality objective(s) for a particular use or uses. This practice allows for calculation of WQEs with user selected  RSDs less than 30 %.

1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.