Learn the difference between cp cpk and pp ppk formulas and the different. You know, things like cpk, ppk, sigma level, ppm out of spec and so on. When ppk stands for process performance index, cpk stands for process capability index. So in this post i will show you how to calculate ppk using minitab. Qi macros vs minitab comparison compare minitab and qi. Overall cpk only accounts for within subgroup variability. Ppk uses the actual sigma value in its formula so it. Design of experiments doe minitab masters module 5 duration. Your supplier has sent you the process capability charts you requested. Looks like your supplier is really performing for you. Ppk vs cpk a good, clear explanation and how minitab handles certain statistics. Ppk, process performance, often referred to as long term capability, the overall capability indices are associated with the overall sample standard deviation. Some of my colleagues have taken other approaches to explain the difference between ppk and cpk. If i understand your question, the ppk that minitab calculates is the long term process capability which uses the formula you quote for your formula in excel as cpk.
Cp cpk formulas versus pp ppk formulas whats the difference. Data 6 pp input range cell links statistics calculations c4d yield count below lsl count above usl sum protection email for questions and bugs cp cpk. It is commonly used to measure whether the process is capable of meeting a customers requirements. Interesting video on assessing ppk and cpk in minitab. Minitab process capability free tutorial capability.
Please explain why pp and cp are referred to as centralized rates, and cpk and ppk are referred to as unilaterial rates in paragraph 5 of this article. Process capability cp, cpk and process performance pp, ppk. To illustrate the calculation of ppk and cpk, well use a sample data set available in minitab. If a capability index cannot be calculated, minitab inserts an asterisk next to the index in the capability analysis results. Cpk is the shortterm capability of a process, and ppk is the longterm. The real truth behind cpk and ppk capability and potential. As you can see in the above figure, we have considered the three conditions as cp1, cp1 and cp minitab. I then tried all the other different methods rbar, sbar, pooled, and biasedunbiased that minitab provides of estimating sigma and neither one resulted in the same cpk as excel. Minitab process capability free tutorial capability analysis by minitab. Download excel template of cp and cpk, use minitab or excel format to calculate the capability and it index. Yes, we have minitab but we have so many results that all have different spec limits so its almost impossible to go and give the limits manually. Readers, today we will guide you to learn a new topic as how to do the capability analysis by minitab18. Minitab started using pooled standard deviation to calculate cpcpk, and. The main difference between pp and cp on one side and ppk and cpk on the other side is whether we use complete set of data for calculation pp and ppk where we calculate real performance of the system, or we use sample preproduction, batch, logical subgroups where we calculate capability of the process.
Process capability study i process capability cp and cpk i cp cpk and pp ppk about this video. Its a good question, especially since many practitioners default to using cpk while overlooking ppk altogether. How cpk and ppk are calculated, part 1 minitab blog. When i put all this into minitab and group by lot, i get a cpk value of 1. Histogram cp cpk cheat sheet qi macros spc software for. Capability suite creates six charts including histogram, control charts, probability plot, values plot and capability plot. Minitab started using pooled standard deviation to calculate cp cpk. The following training module has been created in order to increase the interpretation of quality control using minitab. Generally you use this when a process is under statistical control. Essentially, it is an evaluation of how well a process performed. Its a good question, especially since many practitioners default to using cpk while overlooking ppk.
The supplier has produced some very nice charts, obviously generated with some fancy software package and, of course, with all those accompanying statistics. Z score and process capability six sigma study guide. This often happens with a mature process that has been around for a while. When a process is in statistical control, ppk and cpk are approximately equal. The capability indices cpk, ppk, and cpm are all greater than 1. Minitab s capability analysis output gives you estimates of the capability indices ppk and cpk, and we receive many questions about the difference between them.
May 2014 your supplier has sent you the process capability charts you requested. This document describes the differences between cpk and ppk values. And here is where the danger lies in only reporting cpk and forgetting about ppk like its george michaels lesserknown bandmate no offense to whoever he may be. Histogram calculates cp, cpk, pp, ppk, sigma and twenty other metrics using your data and spec limits. As seen on the attachments, cpk is calculated as 0,99 in excel with formulas but on minitab it is calculated as 1,16 but ppk.
Ppk vs cpk a good, clear explanation and how minitab handles. Ppk vs cpk a good, clear explanation and how minitab. I have only recently just like your postings ironically been trying to figure out the difference between cpk and ppk. Z score and process capability are used together to get a view of your process. If you see an asterisk in your output, it may be due to one of the following reasons. Process capability study using minitab cp cpk duration. When you examine the relationships for both cpk and ppk. Maschinenfahigkeit prozessfahigkeit cpk wert cmk wert mfu. I have some measurements from the production datas and analyse it on minitab and on an excel spreadsheet which calculates the cpk. Process capability study i process capability cp and cpk i.
Process capability uses the process sigma value determined from either the moving range, range or sigma control charts. Michelle paret already wrote a great post about the differences between cpk and ppk, but it also helps to have a better understanding of the math behind these numbers. Cp, pp, cpk, and ppk are capability and performance indicators for your process. Decision of process capability analysis download format. Interpretation because samples of data are random, different samples collected from your process are unlikely to yield identical estimates of a capability index. Process performance, ppk, is important because it indicates how the actual process performed over a period of time.
What is the difference and why it is important medical. I am in the process of evaluating the qi macros 30day trial, and so far i am very pleased with its capabilities. Use with the total population when testing the performance of a process. Overall capability for normal capability analysis minitab. Pdf process capability and capability index cp and cpk. A read is counted each time someone views a publication summary such as the title, abstract, and list of authors, clicks on a figure, or views or downloads the fulltext. Why not download a qi macros free trial and start doing it correctly immediately. Minitab will never duplicate the results stated as cpk. Pp and ppk will be equal when the process is centered on its target value. Capability statistics for betweenwithin capability sixpack. The calculation of cpk in minitab takes in to account the effect of time ordering by using the observed level of variation taken from a moving range, range or sigma control charts. So in this post i will show you how to calculate ppk using minitab s default settings when the.
If you need proof of machine or process capability. In my opinion, i tend to agree with qs9000 since the cpk usually greater than ppk. Interpretation because samples of data are random, different samples collected from your process are. In the short sense, cpk and ppk are about process capability and process performance, respectively. Capability cp, cpk, pp, ppk, analysis in minitab explained. Back when i used to work in minitab tech support, customers often asked me, whats the difference between cpk and ppk. Minitab s help indicates that a difference between within and overall capability is an indication. Explained in this video are topics mentioned as below. I may be mistaken, but the manual seems to indicate that cpk. This video is from our blog series on process capability and it takes a look at two common process capability indexes cpk and ppk and which one is better. The truth is that these statistical indices are much more than that, and it is. Some of my colleagues have taken other approaches to explain the difference between ppk and cpk, so i wanted to show you how they differ by detailing precisely how each one is calculated. In technical support, we frequently receive calls from minitab users who have questions about the differences between cpk and ppk.