Standard Reference Materials: Statistical Aspects of the Certification of Chemical Batch Srms (Classic Reprint) - Tapa blanda

Sinopsis

Excerpt from Standard Reference Materials: Statistical Aspects of the Certification of Chemical Batch Srms

Stratified random sampling is used to select bottles for analysis. The sampling is done at random to ensure validity of inference to the rest of the batch. It is stratified to ensure that bottles are selected from the entire range of the population. Because material heterogeneity may be a problem for each srm, it is essential to include bottles from the entire preparation process and fillsequence, which strictly random sampling does not guarantee. For example, if the blending is done in sub-batches, these sub-batches must be represented in the group of bottles to be analyzed so that the null hypothesis of no difference between sub-batches can be tested once the measurements are made. Also, since bottle filling can introduce trends, either because a solvent evaporates, a dry material absorbs moisture, or an analyte volatilizes, the chosen bottles must span the fill sequence.

When multiple chemical methods are used, the bottles are assigned to each method so that each gets a group spanning the entire fill sequence. This way, if there is a trend with fill sequence, this trend will not be confounded with between-method differences.

To select a stratified random sample of 11 bottles, the population of bottles is divided into 11 equal. Sized groups corresponding to sequential intervals of the fill sequence. One bottle is selected at random from each of the 11 groups, and this provides the group of bottles to be analyzed. Sometimes, the chemists feel strongly that the first and last bottles filled should be analyzed. In this case, after the first and last bottles are pulled, the remaining bottles are divided into n-2 equal sized groups and a stratified random sample of n-2 bottles is chosen.

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