Calculating Your Working Tolerance: A Guideline

In accordance with ISO 17025, every lab needs to calculate its working tolerance. Here are some hints on how to determine your own specific working tolerances.

In order to determine the individual measurement-uncertainty estimation, every laboratory that works according to the ISO 17025 standard should prepare a control chart for each reference material, and also for every lab operator.

The recommended proce­dure to define the lab’s working tolerance is to calculate the standard deviation of the measurements of the standard, and state the error tolerances required by your lab. For this, consider the con­fidence in­ter­val that you need:

• A two-fold standard deviation will yield a 95% confidence in­terval.
• A three-fold standard deviation will yield a 99% confidence interval.

The defined confidence interval should be marked on the control chart as the upper and lower limit.

The standard should be measured regularly, preferably on a daily basis. The instructions can be taken from the corresponding standards or internal specifications. Wherever applicable, in the case that the method covers a larger measuring range, two or more standards in the range of the normal results should also be determined.

In our Spectroquant^® Test Kits, we state the accuracy of the respective method on the certificate of quality and in the package leaflet. This can be used to facilitate the estimation of the working tolerance. The working tolerance also depends on the optical path length of the cells that are used.

The accuracy is calculated on the basis of the mean value of the 95% confidence interval of the respective test; this has been calculated over many years of experience along with the specified blank error. The blank error for the reference cell is stated in the certificate for each test kit batch.

These reference materials can be used to check all photometric methods (standards and test kits) and can also be used for non-photometric methods. For non-photometric methods, you will need to perform your own measurement-uncertainty estimation. Guidelines or working tolerances of the type that we offer for our test kits are not available for other methods or for test kits supplied by other manufacturers.