Sampling and Subsampling

Different Approaches to Sampling

The following approaches may be specified in the sampling plan:

Random - Random sampling means that any portion of the sample population has the same probability of being taken. Random sampling is often used for production operations that are continuous. It is also used with constraints, such as a the collection of a random sample during the first, middle, and last third of a production lot that must be analyzed separately to determine if the lot is homogeneous.

Systematic - Systematic samples are collected at predetermined intervals that are defined in the sampling plan.

Stratified - Stratified sampling involves specification of depth, size, color, or some characteristic that must be considered in meeting the objective of the analysis.

Sequential - Sequential sampling is often used to determine if a product meets specification. Initially, samples are pulled in a ‘systematic' fashion and the data is evaluated. If the product is well within specification, no more sampling occurs. If the product is near the specification limit (i.e. - the mean is in specification, but the uncertainty of the measurement goes beyond the specification range), then more samples are pulled to lower the uncertainty and to determine if specification is realized.

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