Statistical Thinking to Improve Quality

This posting continues the resin output variation example described to illustrate the Hoerl-Snee process improvement strategy.   We take this example from Britz et al (2000).   It also appears in Hoerl and Snee (2002).

Having removed the special cause, the Ricoh team focused on output quantity variability.   A histogram displays this variability, and the following figure shows recent output data.  This histogram displays an unexpected pattern indicating a combination of two underlying distributions for the output quantity.   Notice the peaks at 4284 and 4308 kg.

The process flowchart appearing in the previous posting suggested that these two component distributions were due to the split after phase 2 into two separate lines, i.e., lines A and B.   The following histograms shown below confirmed this difference.   The output from line B was consistently lower than line A.   Based on the needs of their customers, the team established the limits shown in the histograms, i.e., 4300 kg ± 5 kg.

Clearly, the variation in output quantity is excessive.   Next the team conducted a brainstorming session to document their collective thinking on potential causes of excessive variation and differences between the two lines.   The following cause and effect diagram shows the result of this session.

The next posting will describe the investigation based on the potential causes shown above.  
Note that the improvement process is iterative. Gather data, identify special cause, gather more data, notice differences, and then conduct brainstorming session.   This improvement strategy looks more like Shewhart’s Plan-Do-Check-Act (PDCA) than the DMAIC steps recommended for Six-Sigma projects.   Also, the team didn’t adopt a specified target until after two data analysis steps.   That is, their Define step occurred in their second PDCA cycle.

References

1. Britz, G. C., D. W. Emerling, et al. (2000). Improving Performance Through Statistical Thinking. Milwaukee, WI, ASQ Quality Press.
2. Hoerl, R. and R. D. Snee (2002). Statistical Thinking - Improving Business Performance. Pacific Grove, CA, Duxbury.