My recent columns have emphasized the need for critical thinking to understand the process that produces any data. By just plotting data in their naturally occurring time order, many important questions arise about the data’s “pedigree” (a term coined by my respected colleagues Roger Hoerl and Ron Snee), including question one: “How were these data defined and collected?”

That simple time plot can easily be converted to a run chart—a time-ordered plot of data with the median drawn in as a reference line. This helps to answer question two: “What was the state of the process that produced these data?”

Two intuitively simple rules can be applied to the chart to determine whether a process has exhibited significant shift during the plotted period:

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