An SPC (statistical process control) tool that is a modification of either the I-MR or sometimes the XBar-R/S Chart tool. Use the deviation-from-nominal (delta) chart for low-volume conditions in which a family of parts shares the same process. For example, machining custom shaft diameters between 1.000 and 2.000 inches is all done on the same equipment, but with frequent changes in specified diameters (we might run from 10 to 20 different diameters in a shift). We can measure the individual readings and subtract the nominal (blueprint) diameters from them. Then, we could then use an I-MR chart to plot these deltas. (See I-MR for details.) If each run of a specific diameter is large enough to collect data using rational subgroups, then a delta Xbar-R/S chart is preferred.
|When to Use||Purpose|
|Pre-project||Assist in project selection by identifying outputs that exhibit high common-cause variation, frequent special causes, unstable variation, or other symptoms that point to the need for improvement.|
|Start of project||Verify process stability when performing a baseline capability analysis.|
|Mid-project||Investigate effects of input variables on the process output over time.|
|Mid-project||Verify process stability when performing confirmation runs after implementing improvements.|
|End of project||Verify process stability after implementing controls to obtain a final assessment of process capability.|
|Post-project||Control inputs to the improved process after the project is complete.|
|Post-project||Monitor the output of the improved process after the project is complete.|
Continuous Y data, in which Y is the deviation of each sampled unit from its nominal specification.