While the computations for a process behavior chart are completely general and very robust, the secret to using a process behavior chart effectively lies in the art of rational sampling and rational subgrouping. Rational subgrouping was the topic of last month’s column. Here we shall look at the broader topic of rational sampling and the bearing it has on the effective use of process behavior charts.

The first axiom of data analysis

No data have meaning apart from their context. Outside of the statistics classroom, data are always generated by a process or system. They may be the result of an experiment where we manipulate inputs and observe outcomes, or they may be the simple byproduct of ordinary operations where the values are observed as they occur. Regardless of the origin of your data, you have to know their context before you will know how to analyze and interpret them. Thus, the collection of data, the analysis of those data, and the interpretation of the results of the analysis will all depend upon the context for the data.

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