Sigma Level and Capability - What are Ppu, Ppl, Sigma Level, Sigma Capability

[fed-up]

Hi

I was just looking through some posts to get a better understanding of Process Performance (Pp & Ppk) and Process Capability (Cp & Cpk). But I couldnt understand by what is meant by the following:

1. Ppu
2. Ppl
3. Sigma Level
4. Sigma Capability

Thanks any help will be greatly appreciated!

 

[Miner]

Re: Sigma level and Capability

Ppu = (USL - Xbar)/3s
Ppl = (Xbar - LSL)/3s
Where s is the actual total standard deviation calculated using the traditional formula that you learned in high school (squares, square roots and n-1). Ppk is the smaller of the two numbers.

Cpu and Cpl are the same formula as Ppu and Ppl, except s is the estimated standard deviation calculated using Rbar/d2. Cpk is the smaller of the two numbers.

See post #3 of this thread for an explanation of sigma level/sigma capability (same thing).

 

[fed-up]

Re: Sigma level and Capability

Hi

Thanks for the quick info. That certainly clarifes a lot.
Do u by any chance know how to calculate sigma level using ppm or dpm?

Thanks

 

[Tim Folkerts]

Re: Sigma level and Capability

Ppk and Cpk both use the equation

|distance from mean to the closer spec limit| / (3*st dev)

The difference is that Ppk collects all the data into one big group and uses that to determine the st dev. Typically this would be the usual sample standard deviation (i.e. STDEV in Excel).

Cpk uses subgroups to determine the st dev. There are various ways to to this. The most common way is the one Miner mentioned: Rbar/d2. There is another way using Sbar. Personally I perfer this second method because it is more accurate, but it is a bit more difficult to do "by hand".

Cpk only looks at variation within each subgroup, while Ppk looks at all variation, so Cpk will have less variation. This means that the standard deviation for Cpk will always be less than or equal to the standard deviation for Ppk. This further means that Cpk itself will be greater than or equal to Ppk.

 

[Miner]

Re: Sigma level and Capability

Hi

Thanks for the quick info. That certainly clarifes a lot.
Do u by any chance know how to calculate sigma level using ppm or dpm?

Thanks

Yes. This is simply a matter of converting PPM or dpm into probabilities (%'s) then looking up the corresponding z-value from a z-table or using a program such as Minitab. The z-value is the same as the sigma level.

 

[fed-up]

Re: Sigma level and Capability

How do u calculate sigma capability and what is meant by it?

Also I understand by what is meant by 1.5 sigma shift. But what i dont get is where did the 1.5 come from?

 

[Miner]

Re: Sigma level and Capability

The z-value = sigma level = sigma capability.

I recommend that you don't worry about the 1.5 sigma shift. It is highly controversial and not worth the effort. It is an attempt to explain what happens to a process long term. However the amount of drift depends on the process and the type of control method used. It may not always be 1.5 sigma. If you want long term sigma capability, simply perform a long term capability study and convert Ppk to long term sigma.

 

[Mark Paul]

My customer says that Ppu and Cpm are interchangable. Regardless, if they want a Cpm>1.30, what is the same value in Ppu?

 

[Miner]

My customer says that Ppu and Cpm are interchangable. Regardless, if they want a Cpm>1.30, what is the same value in Ppu?

Absolutely wrong!

Without going into the details of the equations, I will address the essentials:

• Ppu is driven by the difference between the process mean and the upper spec limit.
• Cpm is driven by the difference between the process mean and the Target.

These are two entirely different things. You CAN NOT convert Cpm to Ppu because they measure two entirely different things. While you can convert from Celsius to Fahrenheit, you cannot convert from Celsius to Pascals.