A useful tool in Six Sigma initiatives is the Multi-Vari Chart and sampling plan. These can greatly help your project. Here’s the step-by-step procedure for pulling intermittent data from a running process:
Select or establish a continuous-type data measurement of process output performance.
This scale may be in units of time, dollars, inches, grams, but whatever it is, it must be a continuous data type.
Explore the historical values of your selected output metric to understand what the magnitude of variation has been in the process.
After you begin multi-vari sampling of your process, you continue until you’ve observed approximately the same magnitude of variation that you’ve seen historically. That way, you’re sure to have monitored the process long enough to have captured the activity in the input factors that is driving variation in the process output.
Define what constitutes a unit in your multi-vari study.
Remember that your defined unit must allow two or more measurements of the process output in different “locations” within or on the unit.
Collect two to five measurements from within the unit defined in Step 3 on three to five consecutive units.
Allow some time to pass — enough that potential factors have a chance to exert new influence on the process.
Repeat Steps 4 and 5 in three to five consecutive-unit intervals until you’ve captured at least 80 percent of the historical process variation.
Simply compare the range of the historical data to the range of the multi-vari data. If they’re approximately equal, you’ve captured enough multi-vari data. If not, keep collecting.
Create a multi-vari chart and analyze and interpret the chart for a primary source of variation.