Attribute Charting - Where does Cp and Cpk come into the equation?



Let me start by saying statistics is something fairly new to me. Let me give some background on how we inspect to make sure I am moving in the right direction.

All inspections we perform is on a pass / fail method. We track our bad product as defective, not defects. Run quantities and sample size quanities are variable.

I am using a p chart to track the data. Once graphed I can show stability. I am struggling with how I demonstrate capability. Is this where Cp and Cpk come into the equation? If so, how?

Defining what the USL and LSL is an unknown to me. I am assuming the LSL is 0, as you can not have less than 0 defects. However when I enter this into the Cpk analysis it shows that I am out of spec on the low side. Not possible as 0 defects is perfect, not out of spec. When I enter the USL, I have chosen 3 percent (not the goal, but anything lower at this time would turn people off to SPC). In order for me to attain a Cpk of 1.33 with 3 percent defect I have to attain a sigma level of greater than 4, with a DPM of 30, which is obvoisly not 3 percent. Have I not entered data correctly somewhere or have I made a mistake somewhere else?

Any advice anyone can give me is greatly appreciated.



Since there are no specification limits for attirbutes, attribute data cannot be used to calculate a Cpk value.

However, you might try calculating the equivalent Cpk.

For example, say your average fraction nonconforming (p-bar) is .00023 or .023%.

Find .00023 on the Z table, which gives you a Z score of 3.50.

Divide 3.50 by 3 for an equivalent Cpk of 1.16.

To hit an equivalent Cpk of 1.33, you will need a maximum defect rate of around 64 per million.


Thanks for your advice Ben. Can you tell me where I can find a z table. I have one, but it leaves large gaps in the percentages. If I am going to use this with any regularity then I need a chart with greater detail.

Bill Ryan - 2007

I messed something up posting the link but if you copy the address and paste it in the search line it works.


Ravi Khare

Z table

If you have a copy of MS Excel handy, you could get the z score of any value at a fine resolution. There are 3 functions related to Normal Distributions within Excel.
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