Theory of Sampling (TOS) course, conducted by professor Kim H. Esbensen (2013) :
Understanding what sampling variation is, and how it is calculated has been a “light-bulb” moment for our analysts after considering the Theory of Sampling principles (TOS). So often we have had a situation where analytical work done can be verified, but our customer still insists it doesn’t meet his expectations. Short of driving the poor analyst crazy with re-work exercises, which usually only produces the same “incorrect result”, I now have an avenue of action to take that allows us to guide the customer and analysts as to how they should take representative samples. This is decidedly more welcome than always having to say “Take it back to the lab – repeat the analysis”.
Much time is spent determining Total Combined Uncertainties for Analytical Methods under Validation, however very little attention is given to the sampling errors and the challenges heterogeneity poses to this issue, and the sampling errors dominate over their analytical cousins. Also, using variograms as a quality control tool for monitoring is a very powerful technique that could help process controllers explain variations that occur on their product lines.
I found that the new international standard DS 3077 (2013) and in particular its use of illustrations and industrial examples captured the true enormity of the many types of Sampling Errors and helped to conceptualize the Theory of Sampling principles in a striking visual way, making it easier for a typical chemical analyst to relate to the scenarios described. After all, we have to isolate the smallest aliquot for analysis – as demanded by highly sophisticated analytical instrumentation. It is therefore highly surprising that the one area of greatest error affecting an analysts results, is the same topic largely ignored in Analytical Chemistry/Science Training programs. This gives rise to brilliant analytical results, extremely precise, but for non-representative samples for which accuracy with respect to the lot is not accounted for! I just wonder how many questionable results have been released by laboratories all over the world and they don’t even know this …
Dr. Melissa C. Gouws