Multi-Vari Chart

This posting describes the Multi-Vari Chart which is a tool for use in the seventh step, Study Cause and Effect, of the Hoerl-Snee Process Improvement Strategy.  The posting defines the chart and illustrates its use.

The Multi-Vari Chart graphically shows variation of a quality characteristic for multiple factors.   The purpose of the chart is to permit identification of the factor or factors having the greatest effect on variability.

Recall the example in the previous posting taken from Breyfogle (2003, page 389).  An injection molding process produced plastic cylindrical connectors.   The example included data from a sample of two parts collected hourly from four mold cavities for three hours consisting of measurements at three locations on the parts.  The three locations are bottom, middle, and top.  We want to display the variability by location, cavity and part.  The following figure shows averages over the three hours by location, cavity and part.   The figure shows that cavities 2,3 and 4 had larger diameters at the ends (top and bottom) while cavity 1 had a taper.   Thus, cavity and location have an interacting effect.

In this example, the Multi-Vari chart showed interactions among categories affecting variability.   In the previous posting, the Box Plot shows variation within a category, i.e., a cavit.

References

1. Breyfogle, F. W. (2003). Implementing Six Sigma. Hoboken, New Jersey, John Wiley & Sons, Inc.