Designing A Sampling Plan
A sampling plan must support the goals of an analysis. In characterization studies a sample’s purity is often the most important parameter. For example, a material sci- entist interested in the surface chemistry of a metal is more likely to select a freshly exposed surface, created by fracturing the sample under vacuum, than a surface that has been exposed to the atmosphere for an extended time. In a qualitative analysis the sample’s composition does not need to be identical to that of the substance being analyzed, provided that enough sample is taken to ensure that all components can be detected. In fact, when the goal of an analysis is to identify components present at trace levels, it may be desirable to discriminate against major components when sampling. In a quantitative analysis, however, the sample’s composition must accurately represent the target population. The focus of this section, therefore, is on designing a sampling plan for a quantitative analysis.
Five questions should be considered when designing a sampling plan:
· From where within the target population should samples be collected?
· What type of samples should be collected?
· What is the minimum amount of sample needed for each analysis?
· How many samples should be analyzed?
· How can the overall variance be minimized?
Each of these questions is considered below in more detail.
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