# Normality of distribution prior to Cpk and PpK

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#### Kennydu1981

The supplier was required to do the normality distribution study prior to initial process capability study, but they tell me they have performed Ppk stuy which is unnecessary to perform normality distribution study. and the normality distribution study will performed only prior to cpk study. Can anyone explain the difference of normality distribution study prior to Cpk and PpK?

#### bobdoering

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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.

#### Kales Veggie

##### People: The Vital Few
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 are making an interesting point. What is the reason for non-normal, uniform distribution in precision machining? What tolerances are typical for such a process?

#### bobdoering

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I hate to go through the whole explanation in this thread, since it was really about Cpk and Ppk. The details were laid out in the following thread:

To take into account the tool wear during calculating control limits (You can start at post #5)

My definition of "precision machining" is where the machining process is controlled to where the only significant variation is the tool wear. So, it really is not the tightness of the tolerance - any tolerance would apply.

But, to answer capability, the capability of a uniform distribution is (USL-LSL)/(UCL-LCL). Kooky, huh?

If you have any further questions after reviewing that thread, pass them along!

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