LSL of Zero...or not
LSL of Zero...or not
(OP)
I'm doing a capability study on the flatness of a group of parts. The IDI indicated the Weibull as the best plot use. The LSL on flatness is always 0. What I noticed is if I enter a 0 for the LSL I get a Ppk of 1.08. If I leave the LSL blank I get a Ppk of 2.72. Why this difference and which one should I use.
Thanks folks,
Thanks folks,
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech





RE: LSL of Zero...or not
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LSL of Zero...or not
I'm using Minitab 16 and an IDI is Indvidual Distribution Identification. It's a tool into which you can bring your data and it will apply the different distributions and transformations to it to see which it fits best. The you can do your analysis using the correct plot.
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: LSL of Zero...or not
Then you'll know what Minitab thinks a blank LSL means (my guess would be -USL, but that is wrong, that would still give the same Ppk. That would be the likely case for process capability (Cpk)). Then shoot the minitab developer for making unwarranted assumptions.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LSL of Zero...or not
I must admit I've rarely had to think about single sided distributions, the hack answer of reflecting it about the mean seems awful.
In reality you are concerned with what proportion exceed your USL. Quite obviously you don't have a normal distribution, so any assumptions about 3 sigma =99.6% or whatever are misleading. I can't be bothered to research it for you but one sided distributions should be in any reputable text. In the bad old days the statisticians used to transform the x axis, using any old function that worked, until they had a normal distribution, and then they could just use the normal tools that other people with brains had worked out for them.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: LSL of Zero...or not
Thank you again for your help.
John Acosta, GDTP Senior Level
Manufacturing Engineering Tech
RE: LSL of Zero...or not
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?