One hundred years ago this month, Walter Shewhart wrote a memo that contained the first process behavior chart. In recognition of this centennial, this column reviews four different applications of the techniques that grew out of that memo.

The first principle for interpreting data is that no data have any meaning apart from their context. Context tells us what type of analysis is appropriate, and how to interpret the results of our analysis. The following will illustrate this principle.

Observational studies

The immediate and most common use of Shewhart’s technique is with observational studies. Here, either an Average and Range chart or an XmR chart is used for the real-time, sequential analysis of a continuing stream of operational data. A baseline period is used to compute limits that define what a predictable process is likely to produce, and then these limits are used with additional data as they become available. Every time we add a point to our chart, we perform an act of analysis; the chart asks, “Has a change occurred?”

Every time we add a point to our chart, we perform an act of analysis—the chart asks, “Has a change occurred?”

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