Short-run capability is run on the assumption that the data is taken from a single period of time. The idea is to view data from a single sample point without any information about time. There is no subgrouping of the data or time information as the results may come from a small batch of products. Because of this, no within standard deviation can be found, this means Cpk will not be calculated and only Ppk will be used.
The Assistant tool for capability analysis helps us find the type of study to run and provides guidance in term of output and preparation of the data. Like all the Assistant tools, the dialog box is presented without any options in order to make the choices simpler and easier to use.
Because the emphasis is on ease of use, with the Assistant tool, we will not use a sample data set for this recipe. Instead, try following the instructions with your own results.
The following instructions show the steps to choose the capability analysis in the assistant:
- Go to the Assistant menu and select Capability Analysis.
- From the decision tree, scroll to Continuous and select the Capability Analysis option at the bottom of the tree, as shown in the following screenshot:
- Change Type of analysis to Snapshot, and then enter your data column in the section labeled Column.
- Enter the lower and upper specifications where indicated, and click on OK.
As the snapshot considers data from a single time point, we do not produce Cpk. This is because Cpk is calculated from within the group variation that we cannot have with a snapshot of the data. Only Ppk for the overall variation of the samples is given.
The Assistant menu's capability tools offer a simplified dialog box to produce capability. If we had used a complete capability analysis, then Assistant would also produce control charts. This generates control charts for Xbar-R, Xbar-S, or I-MR, based on the subgroup's size.
While the Assistant tool does not offer a capability for nonnormal data, it will transform the results with the Box-Cox transformation. Minitab will ask before performing the transformation.
We could generate the same capability measures from the Stat menu tools by turning off the options for calculating within capability and only displaying Ppk.