Tim Folkerts
Trusted Information Resource
Understanding capability indices is a repeated concern here at the Cove. So I thought I would provide my own take on interpreting the common capability indices: Cp, Cpk, Pk, and Ppk.
Rather than start with a theoretical presentation of the equations, my plan is to start with the data. In subsequent posts, I will present data for a hypothetical process. Each set of data is generated to illustrate one or more specific features. By seeing the data and the capability indices, hopefully the interpretation will be more concrete, more intuitive.
Unless other wise stated, the following will be true of all the data:
Comments are, of course, welcome. If there are any particular situations you would like illustrated, let me know, and I can try to work it into the series.
Tim
Rather than start with a theoretical presentation of the equations, my plan is to start with the data. In subsequent posts, I will present data for a hypothetical process. Each set of data is generated to illustrate one or more specific features. By seeing the data and the capability indices, hopefully the interpretation will be more concrete, more intuitive.
Unless other wise stated, the following will be true of all the data:
- The process has specification limits at +/- 5
- Data is generated from a normal distribution
- Data are collected into subgroups of 4
- Calculations will be based on 30 subgroups (occasionally 100 subgroups)
- Processing is done with Minitab
- Data will be screened; the data is randomly generated, but data sets that are "too unusual" will be discarded in favor of data that more clearly illustrates the desired features.
Comments are, of course, welcome. If there are any particular situations you would like illustrated, let me know, and I can try to work it into the series.
Tim