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#### DavidDeng

Yesterday and today I have discussion about a puzzled problem below.

During the initial study phrase of APQP,we have to do SPC(usually, XbarR diagram).

Speaking of XbarR diagrma, I thought these formulas related with XbarR diagram are all based on the normality distribution study:

UCL(Xbar)=Xbarbar+A2Rbar

LCL(Xbar)=Xbarbar-A2Rbar

CL(Xbar)=Xbarbar

UCL(R)=D3Rbar

LCL(R)=D4Rbar

CL(R)=Rbar

Cpk=min{[( USL-Xbarbar)/3SIGMA(Rbar/D2)],[(Xbarbar-LSL)/3SIGMA(Rbar/D2)]}

as a result,before u begin to caculate the Cpk u had to judge whether all ur data are under the normality distribution.

I got the supporting theory from the page 57 of the AIAG SPC (version 1).it reads:

The individual measurements from the process conform to the normality distribution.

There are many techniques for assessing the capability of a process that is in statistical control.Some assume that the process output follows the bell-shaped normality distribution.If it is not known whether the distribution is normal, a test for normality should be made such as reviewing a histogram,plotting on normal probability paper,or using more precise methods(...).If nonnormality is suspected or confirmed, more flexible techniques should be used,such as data transformation to 'normalize" the distribution...

furthermore,I thought we couldn't caculate any one of these control lines on the specified sheet until we do the normality test to the collected data.

by now,I didn't find any supporting theory and/or evidence from any of an authoritative technical manual(Eg. AIAG SPC)

So, I look forward to the help from all of u.Pls be so kind to do that.

thanks million!