CpK For Attributes Data
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 This topic has 8 replies, 7 voices, and was last updated 15 years, 4 months ago by Pramod Chand.

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June 11, 2001 at 4:00 am #27393
My perception is that, there is not way to calculate capability for attributes data, a process under good capability will show no defects in a time period, or zero defect found in the samples reviewed by our inspector.
Please give your comments.
Bye
0June 12, 2001 at 4:00 am #66984
PraneetParticipant@Praneet Include @Praneet in your post and this person will
be notified via email.Hi Paul
There are two parts to your question
(a) There is way to calculate Process Capability for attributes data. U can use either the Binomial Distribution or Poissons distribution for the same.Binomial data
Binomial data is usually associated with recording the number of defective items out of the total number of items sampled. For example, if you are a manufacturer, you might have a go/nogo gauge that determines whether an item is defective or not. You could then record the number of items that were failed by the gauge and the total number of items inspected. If you are an assembler, you could record the number of parts sent back due to poor fit in the assembly process and the total number of parts purchased. Or, you could record the number of people who call in sick on a particular day, and the number of people scheduled to work that day. These examples could be modeled by a binomial distribution if the following conditions are met:·Each item is the result of identical conditions.
·Each item can result in one of two possible outcomes (“success/failure”, “go/nogo,
” etc.).
·The probability of a success (or failure) is constant for each item.
·The outcomes of the items are independent of each other.In so far as the second part of ur question is concerned then as long as you have a stable process then you will be having rejections as per your established process capability limits and you can accordingly design your sampling plans to inspect the components produced by you.
Regards and have a great day
0June 12, 2001 at 4:00 am #67003Paul,
Your thoughts are correct. A capability study using pass/fail (attribute) data consists of estimating the fraction defective observed in the process over a reasonable period of actual production. Capability indices are typically not calculated with attribute data. However, large samples having np>5 could conceivably produce resonable estimates of Cp and Cpk if you set specifications for minimum and maximum defective rates. These specifications are rarely, if ever, set.
The typical way of conducting an attribute capability study uses the fraction defective estimate with the computed upper confidence bound based on the binomial distribution. Making this estimate allows you to estimate the worst case defective rate expected from the process. A very conservative estimate indeed! You would report this number to management in support of a capability investigation. If you’ve ever done a confidence interval calculation, then you know the sample size is important in estimating the upper confidence bound. In previous, discussions I provide approximate sample sizes required to confidence claims supporting zero observations of defects. Give them a look for additional detail.
Good luck,
Ken
0June 17, 2001 at 4:00 am #67088Do we have to go through all that complexity? Doesn’t the centerline represent your capability?
0June 18, 2001 at 4:00 am #67094RR,
The centerline provides you with only 50% confidence in the estimated quantity. If 50% is good for you, then the centerline is enough!
Ken
0June 26, 2001 at 4:00 am #67277
Paul E.Participant@PaulE. Include @PaulE. in your post and this person will
be notified via email.Ken, thanks for yor inputs.
Paul.
0June 26, 2001 at 4:00 am #67279
Paul E.Participant@PaulE. Include @PaulE. in your post and this person will
be notified via email.Praneet, thank you so much for your comments
Paul.
0September 1, 2003 at 8:48 pm #89467
ElizabethParticipant@Elizabeth Include @Elizabeth in your post and this person will
be notified via email.When studying capability of a process, is it the same to measure 50 units and calculate Cp and Cpk values as it is to inspect 462 (pass/fail) to guarantee a 95% confidence level that the defect rate is no greater than 1?
0May 3, 2007 at 4:34 pm #155666
Pramod ChandParticipant@PramodChand Include @PramodChand in your post and this person will
be notified via email.My view is that CpK for attributes Data can be measured. It is simple that first you convert this data in systematic defined way and now assign numerals as per their weightage, now this weighted numbers can be used for calculating CpK of attribute data.
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