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 (around its diameter, along its length, or across its surface) 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:
-
- Variability on a single piece
- Piece-to-piece variability
- Time-to-time variability
- The quality characteristic of interest is plotted against upper and lower specifications rather than control limits.
The three panels are interpreted as follows:
| 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 |
Read more about this topic: Multi-vari Chart
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