Statistical Aspects of Sample Preparation

Uncertainty in a method can come from both the sample preparation and the analysis. The total variance is the sum of the two factors  

Usually, the goal is to minimize the number of samples, yet meet a specific level of statistical certainty. The total uncertainty, E, at a specific confidence level is selected. The value of E and the confidence limits are determined by the measurement quality required  

The number of analyses can be reduced by choosing an alternative method with higher precision (i.e., a lower aa) or by using a lower value of z, which means accepting a higher level of error. If the analytical uncertainty is negligible (oa — 0) and sample preparation is the major issue, the minimum number of samples, ns, is given by 

the number of samples can be reduced by accepting a higher uncertainty or by reducing as. When oa and as are both significant, the total error Et is given by 

This equation does not have an unique solution. The same value of error, Et, can be obtained by using different combinations of ns and na. Combinations of ns and na should be chosen based on scientific judgment and the cost involved in sample preparation and analysis. 

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