You
do need to know what your distribution is to calculate any capability (and there a bunch more than just Ppk or Cpk). For example, in precision machining the distribution for diameters and lenghts is non-normal, they are the uniform distribution. Ppk and Cpk are
not the correct calculations for capability.
You can refer to AIAG PPAP 4th Edition:
2.2.11.3 Acceptance Criteria for Initial Study
The organization shall use the following as acceptance criteria for evaluating initial process study results for processes that appear stable.
2.2.11.5 Processes with One-Sided Specifications or Non-Normal Distributions
NOTE: The above mentioned acceptance criteria (2.2.11.3) assume normality and a two-sided specification (target in the center).
When this is not true, using this analysis may result in unreliable information. (and it continues on...)
So, it
is important to know if the distribution is normal.
