Hi,

How can I check the process capability for non normal data

prashant

Good question, Prashant.

Capability analysis for non normal data can be done after data transformation. You may like to try box-cox transformation for capability analysis. Make sure that the specification limits are also transformed.

Related query

How do I check for the stability of non-normal data? And what is the course of action in case we find that the data is not stable?

Stability can be checked using control charts. Although there is some debate on applicability of I-MR chart's usage for non normal data, the fact is that it works well in most cases. Xbar chart shall be ok as normality can be assumed for averages.

Isn't normality a pre-requisite for plotting control charts?

No, as points beyond 3 standard deviation from the mean is considered a special cause indicator for (many of the) other distributions as well. Original usage of SPC was demonstrated for distributions that deviate a lot from Normality.

Hi,

How can we can calculate Cpk and Ppk for the Unilateral Process.

Regards,

Mani

Hi Mani,

If you are using Minitab, you shall notice that use of process capability command provides you with Cpk and Ppk values for unilateral tolerances. This is feasible as Cpk uses the minimum of two values (and one shall exist in case of unilateral tolerance).

Hope this answers your query.