total impurities repeatability

[martinamarie]

Hi I hope someone can help me. I have to calculate repeatability for finished product (total impurities).
I ran 6 samples (spiked at the impurity limit) and three controls (unspiked)

Should the total impurities recovered for the run be the sum of total impurities for the six samples - the average total impurities in the control samples or is it the sum of total impurities in the six samples+the average total impurities in the control samples.

Thanks

Martina

[tom jupille]

Do whatever your protocol or SOP says!

If you have no protocol or SOP for this, then, for heaven's sake, write one to avoid the next person having the same problem/question.

Repeatability, by definition, is run on the same sample. Therefore, for this purpose, you ignore your "controls", since they have different levels from the spiked samples.