Process Capability questions - Calculating Cpu using Minitab

S

sivasankara

#1
Hi everyone, I have a question about process capability here

1-Is it possible for Minitab to just generate Cpu, but no Cp and Cpl? I understand that this means there's no target or LSL. But is there product like this?

2-Is it possible to have Cpu value that is negative (e.g. -0.02). Why?

Thank you very much!
 
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Bev D

Heretical Statistician
Staff member
Super Moderator
#2
1-Is it possible for Minitab to just generate Cpu, but no Cp and Cpl? I understand that this means there's no target or LSL. But is there product like this?
I'll leave this to the Minitab experts...
2-Is it possible to have Cpu value that is negative (e.g. -0.02). Why?
Of course. if you think about the formula teh numerator is the upper spec - the process mean. It is not an absolute value. So any process where the process mean is outside (or beyond) the upper spec limit will result in a negative Cpu.
 

Miner

Forum Moderator
Staff member
Admin
#4
1-Is it possible for Minitab to just generate Cpu, but no Cp and Cpl? I understand that this means there's no target or LSL. But is there product like this?
Yes. If you enter an USL, but no target or LSL, Minitab does the following:

  • Calculates Cpu and Cpk
  • Displays an * for Cpl and Cp
  • Does not display Cpm
 

Bev D

Heretical Statistician
Staff member
Super Moderator
#5
It's nice to know that at least one software package does something in an intuitively obvious way!
 
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