[font=Times New (W1)]I have looked back through the archives for Cp and Cpk on unilateral tolerances, and I still need some help.[/font]
[font=Times New (W1)]We now have a situation with our SPC software when it determines capability for unilateral tolerances. The software supplier has offered to fix this if they can get a definitive ruling on what is correct. So here’s your chance.[/font]
[font=Times New (W1)]Here is an example.[/font]
[font=Times New (W1)]Iron content in Aluminum is 0.25% MAX (upper spec limit). Our data averages 0.10%.[/font]
[font=Times New (W1)]Here is my understanding (please correct me)[/font]
[font=Times New (W1)]This software seems to assign 0 as the lower spec limit. This is hard coded, no option to change it.[/font]
[font=Times New (W1)]Process capability (Cp) should not be determined since there is no tolerance as such.[/font]
[font=Times New (W1)]However, the software can mathematically determine Cp by using 0 to 0.25% as the Tolerance.[/font]
[font=Times New (W1)]I believe Cp has little meaning in the case of unilateral tolerances – am I right?[/font]
[font=Times New (W1)]A better measure would be the Taguchi loss function, since it equates loss(cost) to how far from (close to) ideal you are.[/font]
[font=Times New (W1)]Process Capability Centered (Cpk) is even worse.[/font]
[font=Times New (W1)]The software will take the lesser of AVG- LSL or USL-AVG and produce a “mathematically correct” Cpk value.[/font]
[font=Times New (W1)]I believe this is wrong since it results in a poor Cpk, since our process averages much closer to 0 than to the USL. [/font]
[font=Times New (W1)]I believe the Cpk calculation should only be determined for USL in the case of a unilateral tolerance – am I correct?[/font]
[font=Times New (W1)]Interestingly we also have MiniTab, and it seems to do things correctly (if you set it up right).[/font]
[font=Times New (W1)]Minitab will calculate a Cp if you enter 0 as the lower spec limit. However, if you leave the LSL blank, Minitab will not report a Cp value. This makes sense to me.[/font]
[font=Times New (W1)]Also Minitab will calculate Cpk from the USL if you leave the LSL blank. However, if you enter 0 it will report the lower value.[/font]
[font=Times New (W1)][/font]
[font=Times New (W1)]Looking forward to an intense learning experience[/font]
[font=Times New (W1)][/font]
[font=Times New (W1)]Caster[/font]
[font=Times New (W1)]We now have a situation with our SPC software when it determines capability for unilateral tolerances. The software supplier has offered to fix this if they can get a definitive ruling on what is correct. So here’s your chance.[/font]
[font=Times New (W1)]Here is an example.[/font]
[font=Times New (W1)]Iron content in Aluminum is 0.25% MAX (upper spec limit). Our data averages 0.10%.[/font]
[font=Times New (W1)]Here is my understanding (please correct me)[/font]
[font=Times New (W1)]This software seems to assign 0 as the lower spec limit. This is hard coded, no option to change it.[/font]
[font=Times New (W1)]Process capability (Cp) should not be determined since there is no tolerance as such.[/font]
[font=Times New (W1)]However, the software can mathematically determine Cp by using 0 to 0.25% as the Tolerance.[/font]
[font=Times New (W1)]I believe Cp has little meaning in the case of unilateral tolerances – am I right?[/font]
[font=Times New (W1)]A better measure would be the Taguchi loss function, since it equates loss(cost) to how far from (close to) ideal you are.[/font]
[font=Times New (W1)]Process Capability Centered (Cpk) is even worse.[/font]
[font=Times New (W1)]The software will take the lesser of AVG- LSL or USL-AVG and produce a “mathematically correct” Cpk value.[/font]
[font=Times New (W1)]I believe this is wrong since it results in a poor Cpk, since our process averages much closer to 0 than to the USL. [/font]
[font=Times New (W1)]I believe the Cpk calculation should only be determined for USL in the case of a unilateral tolerance – am I correct?[/font]
[font=Times New (W1)]Interestingly we also have MiniTab, and it seems to do things correctly (if you set it up right).[/font]
[font=Times New (W1)]Minitab will calculate a Cp if you enter 0 as the lower spec limit. However, if you leave the LSL blank, Minitab will not report a Cp value. This makes sense to me.[/font]
[font=Times New (W1)]Also Minitab will calculate Cpk from the USL if you leave the LSL blank. However, if you enter 0 it will report the lower value.[/font]
[font=Times New (W1)][/font]
[font=Times New (W1)]Looking forward to an intense learning experience[/font]
[font=Times New (W1)][/font]
[font=Times New (W1)]Caster[/font]