ONE GOOD IDEA
The 3-D Pareto Chart
A better graph to improve visualization and error response
by William A. Levinson
The traditional Pareto chart is a vertical bar graph that provides a static picture of nonconformance categories and their relative importance. Pareto charts also have been used to track considerations such as profits, sales and operating expenses over time.
A version called a trending Pareto chart shows variance over time by providing one bar per unit of time (for example, month or quarter) per category.1 The format is still, however, two-dimensional.
Extending the data from a trending Pareto chart to a 3-D format, on the other hand, can illustrate not only the relative impact of trouble sources, but also the effect of closed-loop corrective action to mitigate those categories. Figure 1 shows an example with six nonconformance sources, categories A through F, over four months.
The results from the first month, January, look similar to a traditional Pareto chart and indicate that category B is the biggest source of nonconformities, which makes it the first choice for closed loop corrective action.
The drastic decline of nonconformities in category B from January to February shows that the closed-loop corrective action was successful.
The 3-D Pareto chart’s only drawback is that the nonconformance categories cannot be re-ranked in each time period, but the Pareto principle is still apparent in the successive periods.
For February, focus should turn to category C, not only because it has the highest rate of nonconformity for February, but also because its rate for January stands out, as is evident in the chart.
Successful closed-loop corrective action for category C causes category F to stand out clearly in March. Suppression of the problem in category F leaves no obvious major contributor for April, although it may be desirable to address the minor number of defects in categories A and D.
The 3-D Pareto chart supports ISO 9001:2015, clause 7.3, which indicates that all people doing work under the organization’s control are aware of "their contribution to the effectiveness of the quality management system, including the benefits of improved performance."2 That is, workers can immediately see the history of improvement activities in which they may have participated.
As a final note, it is possible to insert a hyperlink into any cell of the spreadsheet that generates the 3-D chart, which means that the spreadsheet can reference electronic quality records such as in-progress or completed closed-loop corrective action projects. If, for example, after corrective and preventive action (CAPA) was taken on defect category B in January, the cell could link to the CAPA report.
1. Pareto Charts, "Showing Improvement," http://tinyurl.com/trendingparetochart, April 29, 2015.
2. International Organization for Standardization, ISO/FDIS 9001:2015—Quality Management Systems—Requirements, Clause 7.3.
William A. Levinson is principal consultant at Levinson Productivity Systems P.C. in Wilkes-Barre, PA. He has a master’s degree in engineering from Cornell University in Ithaca, NY, and an MBA from Union College in Schenectady, NY. An ASQ fellow, Levinson is an ASQ-certified quality manager, auditor, reliability engineer and Six Sigma Black Belt. He is the coauthor of The Expanded and Annotated My Life and Work: Henry Ford’s Universal Code for World-Class Success (Productivity Press, 2013).