Multi-vari chart

In quality control, multi-vari charts are a visual way of presenting variability through a series of charts. The content and format of the charts has evolved over time.

Original concept

Multi-vari charts were first described by Leonard Seder in 1950,[1][2] though they were developed independently by multiple sources. They were inspired by the stock market candlestick charts or open-high-low-close charts.[3]

As originally conceived, the multi-vari chart resembles a Shewhart individuals control chart with the following differences:

  • Variability on a single piece
  • Piece-to-piece variability
  • Time-to-time variability

The three panels are interpreted as follows:[4]

Panel Condition Corrective action
Variability on a single piece Lengths of the vertical lines (i.e., the range) exceed one-half the specifications (or more) Repair or realignment of tool
Piece-to-piece variability Excessive scatter Examine process inputs for excessive variability—lengths of the vertical lines are estimates of process capability
Time-to-time variability Appearance of a non-stationary process Examine process inputs or steps for evidence of shifts or drifts

Recent usage

More recently, the term "multi-vari chart" has been used to describe a visual way to display analysis of variance data (typically be expressed in tabular format).[5] It consists of a series of panels which portray minimum, mean, and maximum responses for each treatment combination of interest rather than for periods of time.

Because it is a two-dimensional representation of multiple dimensions (one for each factor in the ANOVA), the multi-vari chart is only useful for comparing the variability among at most four factors.

The chart consists of the following:

References

  1. Seder, Leonard (1950), "Diagnosis with Diagrams—Part I", Industrial Quality Control (New York, New York: American Society for Quality Control) 7 (1), pp. 11–19
  2. Seder, Leonard (1950), "Diagnosis with Diagrams—Part II", Industrial Quality Control (New York, New York: American Society for Quality Control) 7 (2), pp. 7–11
  3. Juran, Joseph M. (1962), Quality Control Handbook (2 ed.), New York, New York: McGraw-Hill, pp. 11–30
  4. Juran, Joseph M. (1962), Quality Control Handbook (2 ed.), New York, New York: McGraw-Hill, pp. 11–30–11–31
  5. Tague, Nancy R. (1995), The Quality Toolbox (2 ed.), Milwaukee, Wisconsin: American Society for Quality Control, pp. 356–359
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