**FAQ for Nordtest 537 (2017)**

**Here are questions and answers on this handbook**

**Printing error?**

Example A: *BOD with Internal quality control+ a CRM *in part 4 it shows a term sbias2/√n, should read (sbias/√n)2 ?

Reply: This is missing a parenthesis THANKS. The parenthesis is shown in next part of the formula.

**GENERAL**

1 Is this approach ISO GUM compatible?

Reply: This handbook follows the GUM principles of uncertainty and is widely used by accredited laboratories in many different sectors.

2 ‹ Why the uncertainty from day to day variation from control

charts is calculated as u=stdev and is not calculated as u=

stdev/sqrt(n)?. Reply: The s should be calculated based on control values analysed the same way as test samples, i.e. if the test samples are analysed as duplicates (*n =* 2) the control values should also be based on duplicate analyses. Then you do not consider dividing - just use the *s* you obtained

3. In chapter 6.3 Recovery the *u*(vol) is calculated using square root of 3? Reply: If we assume that error of the flask can be up plus or minus 1 % then the standard uncertainty, *u*, can according to GUM for a so called rectangular distribution be calculated by dividing by square root of 3.

**BIAS**

1. **Calculation of u(bias) from a bias limit**

If a method has an acceptable bias limit of say ± 10%, and the data is within these limits this can lead to very high expanded uncertainties in some cases- can this be corrected or is it just the ‘natural’ consequence of high bias levels?

Reply: If instead of applying the measured bias you use the acceptable bias limit in the formula you can regard this as a rectangular distribution (i.e. the bias is anywhere within these limits for all samples within the scope of the method) and then divide by sqrt3 to get a standard uncertainty. If we take data from the EU drinking water directive where max bias is 10 % and max precision is 5 % (2 sRw max 10 %) at the relevant level for many parameters, the expanded uncertainty can then be calculated to be about 15 % relative.

See further details on page 158 in a chapter on measurement quality in a book

2. **How to calculate bias using recovery data**

In section 6.3 Recovery is an example of a validation of 6 *different matrices*. For *each* matrix we have a mean recovery and the overall mean recovery is 96,8 %. We use the same formula as for several CRM. The RMS of these values are 3,44 %. NOTE The guidance given in this section is applicable for test methods that do not include a recovery correction in the procedure for each analytical run. If however you have a fixed recovery in your method then you can validate the method with new experiments and use the same formula. The MUkit software handles recovery where at least 6 matrices are tested.

**SAMPLING**

**1. Sampling uncertainty**

In order to estimate *measurement* uncertainty for a sampling target also the sampling uncertainty should be estimated. Here you can use duplicate sampling according to Nordtest 604. The *measurement* uncertainty can then be calculated by combining the *u*(sampling) with the *u*(analysis) where the *u*(analysis) is estimated from Nordtest 537.