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, though they were developed independently by multiple sources. They were inspired by the stock market candlestick charts or open-high-low-close charts.As originally conceived, the multi-vari chart resembles a Shewhart individuals control chart with the following differences:
- The quality characteristic of interest is measured at two extremes and these measurements are plotted as vertical lines connecting the minimum and maximum values over time.
- The quality characteristic of interest is plotted across three horizontal panels that represent:
- The quality characteristic of interest is plotted against upper and lower specifications rather than control limits.
| Panel | Condition | Corrective action |
| Variability on a single piece | Lengths of the vertical lines exceed one-half the specifications | 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. 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, the multi-vari chart is only useful for comparing the variability among at most four factors.
The chart consists of the following:
- One horizontal panel for each level of the outermost factor
- One cluster of points representing the minimum, mean, and maximum responses for the particular treatment combination, connected by lines for each level of the innermost factor
- In the case of four factors, vertical panels for each level of the next-innermost factor
- As with control charts, the vertical axis depicts the quality characteristic of interest