Hoerl-Snee Example
Resin Output Variation Example (Part A)
Ricoh’s Numazu plant made raw material used as ingredients for copy machine toner. The tolerances for the raw material ingredients were measured in ten-thousands of a gram. They had a team which continually monitored the process to achieve continuous improvement. The team noticed a problem with actual output results compared to theoretical output quantities. The yield ratio frequently exceeded 1.0. That is, the actual output quantity for a batch divided by the theoretical output quantity sometimes exceeded 1.0. The following figure shows a run chart displaying these results. These values were technically impossible, so the team attributed these results to undesirable variation somewhere in the process. Their experience indicated that this variation would degrade finished product quality. They wanted to know the source of the variation and how to eliminate it.
The figure on the left gives a flowchart of the process. Notice that after the second phase the process splits into two lines supposedly with identical sub-processes.
The next step in their investigation was to examine the run chart for stability. The time period in the middle of the run chart had the greatest concentration of values above 1.0. Further analysis showed that a drop in air pressure was the root cause (special cause) for the excessive variations above 1.0. They verified this conclusion, but removing this cause did not completed eliminate the outputs greater than theoretical predictions.
Notice that their first steps were to document (understand) the problem and look for special causes.
References
- Britz, G. C., D. W. Emerling, et al. (2000). Improving Performance Through Statistical Thinking. Milwaukee, WI, ASQ Quality Press.
- Imai, M. (1986). Kaizen, The Key to Japan’s Competitive Success. New York: Random House Business Edition.
