Initial Process Studies - Number of samples - 100 for Individuals Chart?

Stuart Andrews

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Hi folks!
A simple question! When performing Initial Process Studies for QS-9000 should the number of samples be at least 100 for Individuals Charts?

And should the Ppk be above 1.33 or is there a specific reason why using this type of chart should give you low Ppk values.

Your help is appreciated.


Yes, according to QS standards, 100 is the general rule of thumb.

Reference: PPAP manual – Rev 3, pg. 6&7, section I. Quality Indices
Reference: SPC Manual, pg. 31, section A.1.c.

Should the Ppk be above 1.33? Well, if we just go by the standard, a Quality Index of <1.33 doesn’t cut it. 1.33 to 1.67 acceptable but may require some improvement. > 1.67 – rock on. But ultimately it is what you and your customer agree upon.

Stuart Andrews

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Thanks AJLenarz

It's just that I've never dealt with individuals charts before and a supplier to us submitted a PPAP with a sample size of 68 and a Ppk of 0.74.

I just wanted to make sure that there wasn't a special rule with these charts that differed from xbar and rbar charts.
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