Technical Topics Lab Investigation Report (LIR) How to set up a practical and appropriate control chart RSD? Reply To: How to set up a practical and appropriate control chart RSD?

David Woollard
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I assume you have established a control sample with at least 7-10 days of independent results. Don’t use same-day replicates.

These form the basis for future acceptance. Plot them. On the control chart mark the horizontal 2x SD lines and the 3x SD lines (+/-) and the mean.

If a future result exceeds 2x SD then consider it suspicious enough to review the data set. Check the calibration etc.

Reject the run if a future result exceeds 3x SD. Repeat the batch.

The question arises if a new result is incorrectly rejected because the SD limits are too tight.  This requires discussion and a decision by the Quality Manager.  The limits plus mean can be recalculated and put into the control chart. Only do this openly and sparingly.

It only takes a short time to decide whether the control chart is correctly set.

There are cases where a control sample is unstable and changes in composition. Eg The analyte might be unstable or the sample absorbs moisture. The control chart will show this by repeated results lying on one side of the mean. If the changes are slow then they can be accommodated with scheduled updates to mean and SD. If they are quick then the control is useless and needs replacing or upgrading. Freezing control in ready-to-use portions is a common practice.