Process capability
Cp, Cpk, Pp and Ppk with control chart and distribution check
my8data calculates process capability from subgroups: Cp and Cpk from the variation within subgroups, Pp and Ppk from the total variation. Plus the control chart, the Anderson-Darling test and four distribution models for characteristics that are not normally distributed.
- Included in the free account
- No installation, servers in Germany
- Order on invoice, no credit card
| Index | Variation | Meaning |
|---|---|---|
| Cp, Cpk | within subgroups | Potential, short term |
| Pp, Ppk | of all individual values | Actual performance, long term |
Common customer requirement: 1.33 for series production, 1.67 at initial sampling. You set the limits per analysis.
Result
This is what the result looks like
25 subgroups of 5 parts, calculated with sample data. The control charts show a process in control. Cpk 1.93 from the within variation, Ppk 1.81 from the total variation.
Evaluation
What the indices tell you together
If Cpk and Ppk are far apart, the process varies more between subgroups than within them. Both values can be right; they answer different questions.
The x̄ chart with R or s chart shows whether the process is in control. Without a stable process, Cp and Cpk are not meaningful.
Normal, folded normal, Weibull and mixture distribution. For non-normal characteristics the quantile method from ISO 22514-2 applies.
The model tells you how many parts to expect outside the limits. One-sided tolerances are possible; the empty field does not count as zero.
For the machine alone: machine capability with Cm and Cmk. For ongoing monitoring: control charts per AIAG-VDA.
Checked against reference values
The public validation report documents the reference data sets, target values and deviations achieved for the capability indices, plus the cross-check with Minitab.
Pricing
Every method in every plan
All prices excl. VAT. 12-month term, cancellable with 30 days’ notice. Order by PDF form on invoice.
- All methods
- Unlimited calculations
- PDF and Excel export
- Save analyses
- Everything in FREE
- PDF report from your own Excel template
- Excel export with data and results
- Save and reopen analyses
- Personal support from the developer
- Everything in PRO, for every user
- Shared team storage
- Seats transferable when staff change
FAQ
Questions about process capability
What is the difference between Cpk and Ppk?
Cpk uses the variation within subgroups and shows what the process can do. Ppk uses the variation of all individual values and shows what it actually delivered.
How many readings do I need?
For a reliable statement about the process, 125 values are common, for example 25 subgroups of 5 parts over a representative period.
What do I do if the data is not normally distributed?
Choose a matching distribution: folded normal, Weibull or mixture. The index then follows the quantile method from ISO 22514-2, the ppm come from the fitted model.
Can I bring in readings from Excel?
Yes. Copy cells in Excel and paste them into the input table. Comma and point are both accepted as decimal separators.
What does it cost?
The calculation is included in the free account. PDF and Excel export and saving cost €490 per user and year excl. VAT with PRO.
Check it with one of your own data series
Create a free account, paste readings from series production and compare Cpk and Ppk with your current evaluation.