1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.
  2. Hello and Welcome to The Quality Forum Online...Continuing in the spirit of People Helping People !
    Dismiss Notice
Dismiss Notice
You must be a registered member in order to post messages and view/download attached files in this forum.
Click here to register.

How to calculate cpk

Discussion in 'ISO 9001:2008 - Quality Management Systems' started by Qualmx, Oct 24, 2015.

  1. Qualmx

    Qualmx Well-Known Member

    Joined:
    Oct 7, 2015
    Messages:
    186
    Likes Received:
    22
    Trophy Points:
    17
    Location:
    Mexico
    Hi all
    Im facing a new issue im my bussiness and I hope you can help me.
    I work in a print shop ,really a small bussiness, and Im loking for new clients and some of them in the USA asked me what is my cpk index.
    Really I Never have used it.
    In a vague way I know it is an index which tells the control we have in our process.
    I'll start to investigate how can I calculate it.
    Meanwhile could you share your experiences in this regard? , very helpful if these come from people working in print shops (offset,flexography, serigraphy)
    I'll appreciate your help.
    Thanks in advance
    Pd I,m implementing ISO 9001.












    Ver
     
  2. Jennifer Kirley

    Jennifer Kirley Moderator Staff Member

    Joined:
    Jul 31, 2015
    Messages:
    597
    Likes Received:
    432
    Trophy Points:
    62
    Location:
    USA
    Oh dear.

    I am afraid you have been speaking with a person who thinks you are a large print shop with a sampling plan and SPC, both of which are involved with arriving at Cpk. Or maybe the person you spoke with has so little imagination that asking about process capability (Cpk) is all he/she knew what to ask about.

    To put it simply, with capability of 3 sigma (Cpk=1.0), a process will produce approximately 99.73% good product or .27% bad product. Higher Cpk is better as it indicates lower defect rate.

    You do NOT need this extent of statistical analysis for ISO 9001, including the 2015 version as this process capability can reflect results of process controls but it is NOT risk analysis.

    If this customer wants you to provide Cpk I hope they are proposing a very large contract, or else I believe the effort may not bring enough value.

    That said, I do not want to discourage you from implementing sensible statistical analysis. I suggest you review Bev Daniel's Profound Statistical Concepts presentations in the Resources Forum.

    I hope this helps!
     
    Bev D likes this.
  3. Qualmx

    Qualmx Well-Known Member

    Joined:
    Oct 7, 2015
    Messages:
    186
    Likes Received:
    22
    Trophy Points:
    17
    Location:
    Mexico
    Thanks Jenn

    You are right maybe our potential client thinks we are a big business, indeed, we are a 30 people print shop, we have limited production rate.
    So yo mean that the cpk is only useful in large production?

    The decision to use it or not, it is depending of the size of the business?

    In our case, cpk does not apply to us? in which case would be recommended?.

    We produce manuals, labels, brochures,etc.

    Thanks for your help.
     
  4. Jennifer Kirley

    Jennifer Kirley Moderator Staff Member

    Joined:
    Jul 31, 2015
    Messages:
    597
    Likes Received:
    432
    Trophy Points:
    62
    Location:
    USA
    There are resources needed to develop the SPC program and prepare the calculations that arrive at Cpk.

    How large are your production runs?

    What do you do to inspect - how often do you inspect the printed articles for defects? (#times per hundred or per thousand run)
     
  5. Qualmx

    Qualmx Well-Known Member

    Joined:
    Oct 7, 2015
    Messages:
    186
    Likes Received:
    22
    Trophy Points:
    17
    Location:
    Mexico
    We have runs starting from 500 up to 15,000
    we inspect the first piece, additionaly we do intermediate inspects normally at about 20 printouts in order to detect defects.
    If in the intermediate inspects we detect non conformance product, we do what is needed
    to fix the problem.

    We also do a final inspection 5 pieces for every package of 100. we arrange products in packages.
    Hope I have explained well.

    Thanks
     
  6. Bev D

    Bev D Moderator Staff Member

    Joined:
    Jul 30, 2015
    Messages:
    343
    Likes Received:
    343
    Trophy Points:
    62
    Location:
    Maine
    Cpk is intended for continuous measurable data. I suspect that most of what you have is 'attribute' or 'defect' counts?
    if so, then Cpk really doesn't apply. you can try to explain this to that Customer.

    too often Customers ask for Cpk just because that's what they do. There is too little thought put into it...
     

Share This Page