Set up the internal quality control by choosing suitable control samples and set-up control charts with preliminary control limits (preferably *target control limits*) - see Nordtest TR 569 "Trollbok" version 2018 for further guidance, www.nordtest.info

# Version 5.1 2018 has the following news

more focus on target control limits. In cases where the client’s demand is lower than the performance of the method, wider control limits can be set. Since this will results in fewer out of control values we recommend laboratories consider this option – see example 1,2,5 and 7;

long-term evaluation (chapter 10) now discusses changing of control limits and central line in separate paragraphs;

pooled standard deviation

* combined standard deviation is now more correctly called pooledstandard deviation;

* pooled standard deviation is recommended to obtain the standard deviation for range charts;

* an example of pooling standard deviation for sr and sRw from an internal control measuring three replicates in every analytical run, example 10, is added. If all results were used to calculate sRw too low estimate is obtained resulting in too narrow control limits;for range chart the best samples to be used are test samples selected among the samples to be analysed in that analytical run – examples have been updated.

## FAQ - Frequently fory Asked Questions

**Central line - should I set it to the reference value or the mean value?**Whenever possible set the central line to the reference value. However you may have to adjust the control limits - see FAQ2

**Should I adjust the control limits if I use a reference value for the central line?**If the reference value is: 200 ug/L and we get a observed mean of 205 ug/L with a standard deviation of 5 ug/L based on more than 60 measurement over a longer time period shall we then use the mean or the reference value for the central line. Here we strongly recommend to use the reference value so the small bias is clearly shown. NOTE you need to have slightly wider. control limits in order not to get to many out of control situations. In this case the warning limits could be 15 and the action limits 22.5 ug/L.