Cpk vs Ppk: shortterm vs longterm process variation
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- Опубликовано: 27 сен 2024
- When calculating SPC control limits, many find out that even the data they use to calculate those limits already breaks rules within that same reference period. That shouldn't be possible, right? Standard deviation calculations tend to estimate wider variation than what's observed in the sample data.
Well, it is. And it's because of how sigma is estimated: not by calculating the standard deviation over the whole sample, but by estimating it based on the average (moving) range observed.
The same happens with Cpk vs Ppk - with Cpk using that same estimation from range and Ppk calculating over all values in the data set.
That is by design, though, because for SPC and Cpk you want to know short-term variation and specifically try to differentiate between everyday process variations and real process shifts.
#continuousimprovement #sixsigma #spc
Very perfectly explained , Looking forward for such more contents in a world where we only see theoretical or bookish explainations.
Thanks for your kind words - I'm happy to hear that you like my approach to our topic
Great lesson. It took me some time to fully get it, but I eventually got there! This topic is not really very often addressed in a technical way like shown in the video. Tom, you clarified everything. Thank you!
I feel you - this is a tricky thing to grasp. I'm also not a big statistics buff myself, but find that when I understand what the statistics are trying to do/prove, it becomes much easier to understand the maths it uses.
Glad to hear that this video helped your understanding too.
@@TomMentink absolutely! Thanks again Tom
Excelente explanation. Thank you!
@@juanpablogonzalez7000 thanks for sharing that, glad to hear my video is of value to you.
Hi Tom, i have seen a scenario where If PP and PPK fail to achieve the minimum requirement e.g. 1.67 but CP and CPK passes. The big factor is the tolerance is so small it doesn't leave much room for any process variation as PP and PPK is long term. What are your thoughts on this due to tight tolerances?
When Cpk is good, but Ppk is not fully up to your demands (Ppk 1.67 is pretty high, mind you), that means your process' average is shifting over time. Maybe there are large and unpredictable differences between material batches, which influences your process outcome, but most other factors should be controllable (not per se easy to control, but that's what you should work on).
In a more recent video, I also explained why I think that for many industries, Cmk >1,67 Cpk >1,33 and Ppk >1,00 is sufficient. You might want to up all of them by 0,33 for more demanding customers, but the tolerances you place on your system do indeed seem very strict. ruclips.net/video/Fi6WPCgwhpg/видео.html
Hi Tom to add to this, after work has been done to optimize factors that can cause variation and you still cant achieve PP or PPK (desired target). what are your thoughts? its up to the customer to then decide whether to except this based on the fact any more improvements would be costly e.g. new tooling etc
@@deanopenn if your Cmk and Cpk are good, but Ppk is mediocre, that should be solvable: it means that there area significant differences between batches and/or operators - focus on those effects.
If your Cpk is already not fully as you'd like it, then going to your customer to renegotiate their expectations is a real option. In the longer run, you'd do better improving your processes (maybe also do a DOE to determine the most stable process parameters, further standardise operating procedures and skill levels, etc.), but isn't always achievable in the short term. And indeed, some customers just ask too much, perhaps even unrealistically high Ppk's for the current state of technology.
You could even go as far as to buy and test a some product by your competition, to assess if they are able to produce at those tight tolerances - if they can, you know you have to catch up, if they also can't it really may be in your customer's expectations.
@@TomMentink thanks Tom, you always answer swiftly and in depth
i knew something would come to my head after i commented haha. in regards to CMK i know you should take consecutive pieces, if the process lets say is a coil of wire and the only time you can get a sample piece is when the coil is full e.g. 500m, how would you go about this scenario? you are looking for the coil diameter for an example
Please be careful with you language as you explain these statistics. ppk 1 is not "horrible".. its likes a 0.1% scrap rate which depending on the situation can be EXCELLENT... you also generalize long term short term as "today" vs the "whole year". This can lead to misconceptions and conclusions from your audience.
You’re right, there is a lot more nuance with these studies, and I take a couple liberties to get the general concept across. I try to balance precision with easy to understand, which is a balance that will never be right for everyone.
On Ppk of 1 being horrible, I agree with you - that was an overstatement: Ppk of 1 to 1,3 is good enough. Thanks for that correction.