cp and cpk
Six Sigma – iSixSigma › Forums › Old Forums › General › cp and cpk
 This topic has 43 replies, 39 voices, and was last updated 14 years, 6 months ago by Bhupendra Nema.

AuthorPosts

July 31, 2001 at 4:00 am #27585
Is there a simple definiton for cp and cpk ?
0July 31, 2001 at 4:00 am #67784There’s a great thread on Cp and CpK here:https://www.isixsigma.com/forum/showmessage.asp?messageID=2035
0August 1, 2001 at 4:00 am #67813
Ranganadha KumarParticipant@RanganadhaKumar Include @RanganadhaKumar in your post and this person will
be notified via email.Cp is the capability of the process to delivery the requirements within the specifications.
Cp is calcuated as (USL – LSL)/6*Std.Dev.
Cp should always be greater than 2.0 for a good process which is under statistical control.
Cpk is the critical process capability something similar to the Cp but the difference is that Cpk looks at both the specification limits, USL and LSL.
Cpk is calculated as below.
Cpl = (Mean – LSL)/3*Std.dev
Cpu=(USLMean)/3*Std.dev
Cpk=Min(Cpl,Cpu)
For a good process under statistical control, Cpk should be greater than 1.5
Cp, Cpk defines the capability of a particular process.0August 2, 2001 at 4:00 am #67841The formulas given by the first person that reply to this message are correct. Both indexes need the specifications. The difference between them is that Cp is a “process potential index” because assumes that the process is centered between the specs or that can be centered. Cpk is a process capability index that measures the proportion of natural tolerances between the center of the process and the nearest spec.
0August 2, 2001 at 4:00 am #67825Thanks that is what I was looking for!!!!!!!!!!!
0August 4, 2001 at 4:00 am #67859
TierradentroParticipant@john Include @john in your post and this person will
be notified via email.cp is a one time measuring process
ex: measuring 150 item at one time
whereby Cpk required to take measurment at specific time frame
ie.: every 1 hr0July 11, 2002 at 8:42 pm #77175Why would my customer specify that Ppk is required over Cpk? We have to acheive a Ppk of 1.67 or greater before they will approve a line for production. I have never used Ppk and I find that it is very difficult to get to a 1.67 value in some cases.
0July 11, 2002 at 9:14 pm #77176
GabrielParticipant@Gabriel Include @Gabriel in your post and this person will
be notified via email.I don’t know your specific case, but ussually is not that Ppk is required OVER Cpk, but some oranizations do not accept the result of a Cpk based on a few consecutive sample from the trial run (what usually happens in a short study for the initial submition) just because in such a short time there is no oportunity to prove stability, what is a prerequisite of Cpk. I support this possition. That’s the same reason (I think) why a Ppk of 1.66 (instead of a Cpk of 1.33) is required (it is a safety margin due the low confidence in such a study). Note that, in such a case, if you dont reach a Ppk of 1.66 you will be allowed to supply anyway, but it will be requested that you take some containment action for the potential non conforming parts while you implement the corrections to reach to a Cpk of 1.33. All that is if your process is a new one. If you have your process already working and with history recorded to show, a Cpk of 1.33 based on those records will be probably accepted. That’s at least my experience.
This topic was widely (and wildly) discussed also in the following threads:
Cp, Cpk, Pp and Ppk
Cp & Cpk against Pp & Ppk0February 23, 2004 at 3:16 pm #95939
rich barberMember@richbarber Include @richbarber in your post and this person will
be notified via email.What does USL an LSL mean and how are they calculated or determined.
0February 23, 2004 at 6:42 pm #95956Rich,
USL and LSL are Upper Specification Limit and Lower Specification Limit. They are not claculated. They are determined by your customer (internal or external).
UCL and LCL on the other hand are your Upper Control Limit and Lower Control Limit on Control charts. These are calculated Using different methods depending on the specific type of chart you use.
A good quick reference to use for these is “The Memory Jogger II” from GOAL/QPC. You can contact them at 18006434316.
I am not now nor have I ever been associated with GOAL/QPC in any way.0February 24, 2004 at 2:25 am #95980
Dr. Steve W.Participant@Dr.SteveW. Include @Dr.SteveW. in your post and this person will
be notified via email.You can have a process with Cp > 2 but all your parts or service are out of spec.! Cp fails to recognize that your process mean has to be in the spec. in order to have a good process. Cp only tells you how good your process can potentially be and it will reach its entitlement only when your process mean is at the middle of the spec. limits. That is why we need Cpk. The difference between Cp and Cpk is not that the latter looks at both spec. limits—Cp looks at them too —you look at the formula you provided. The difference is that Cp ignores process average but Cpk uses it as a key element of its calculation, recognizing the process average has to be within the spec. in order for the process to even have a chance to be good.
0February 26, 2004 at 12:45 pm #96073Flatness is critical to a part my company uses. I have flatness specification of 0.010″. I need to assess capability of the supplier providing the part within that range. How is the best way I can chart my measurement results? I will be taking 3 measurements lengthwise and two widthwise for each part. Since I have a one sided specification, where I want the curve skewed towards the lower side, what is the appropiate way to determine capability. I could always use .005 +/ 0, but that might lead to incorrect analysis results.
0May 31, 2004 at 1:25 pm #100980please send me information about cp,cpk used in statistical process control with example and control charts.
0May 31, 2004 at 3:39 pm #100982
Mike ClaytonParticipant@MikeClayton Include @MikeClayton in your post and this person will
be notified via email.USL and LSL are the upper and lower SPECIFICATION limits used in Cp and Cpk. And they are the Achilles heel of this whole metric. Often they are “American style” limits, which tend to be simply 10% of nominal. Useless in many cases, but common. So Cp and Cpk can be >5 for example in computerized testing, but parts can cause field failures if distribution shifts by many sigmas and are still “in spec.” I recommend no Cpk > 3 allowed! Guardband to that value or tighter to assure that your test system catch big shifts. Your SPC systems can warn at 3 sigma, but are often ignored for containment. Japan used TARGETS for inline SPC and CONTROL limits (calculated from the data, not given by engineers as economically sound guesses). And they used Cpm which does not require SPEC limits, just target and deviations from that.
0May 31, 2004 at 3:52 pm #100983
Mike ClaytonParticipant@MikeClayton Include @MikeClayton in your post and this person will
be notified via email.Single sided specs that originate at ZERO are usually best handled using Cp = Cpk by definition, then if the data looks symmetrical just use the Cpk = Mean/3 sigma of raw data….which is the distance between the Mean and Target (0) divided by 3 standard deviations. Report Cp as the same if required, addding a footnote that Cp is really undefined for single sided limits, but for computer and reporting purposes can be so stated. But if the data is highly skewed, you may need to do some transformation to get more symmeterical distribution (perhaps the logx) then find sigma and mean and transform back again. Most texts show many ways to do this. But remember that increading the sample size may change your results greatly, especially if the extra sites measured are highly correlated. That’s another story.
0June 24, 2004 at 2:26 am #102242Can the cpk ever be greater than the cp?
0June 24, 2004 at 2:30 am #102243No.
Cp assumes the process is perfectly centered — and it rarely is.
Cpk is measured to the closest spec limit.
At best, Cpk is = Cp.0December 8, 2004 at 8:56 am #111995
sam1tim315Member@sam1tim315 Include @sam1tim315 in your post and this person will
be notified via email.Cp = (USLLSL)/6(sigma) = Design_Tolerance/Process_Capability
From the above formula, Cp ONLY tells us whether we have enough tolerance in our design to accomodate imperfection in the manufacturing process. Product design engineers control the numerator while process design engineers control the denominator. Cp DOES NOT tell us if the process is centered within the Design_Tolerance. Thus the need for Cpk which compares the average to the Target Center as shown by the formula below
Cpk = Cp * (1k)
where k = (Target – Process Average)/[(USLLSL)/2]
Cpk is the ACTUAL capability index. It takes into account the actual location of the process mean relative to the specification target.
Ideally k = 0, meaning process is centered at target. Thus it follows that ideally, Cpk = Cp. Now as to the question on whether Cpk can be greater than Cp, i think YES because if you look closely at the formula, k can assume a negative value if Target < Process Average – meaning the process (distribution) is shifted right from the target. I am coming from the assumption that k formula is not enclosed in absolute value operator.
Thus, if k can go negative, we can say that ideally Cpk should be slightly less or slightly more than Cp. We know that process has shifted left or right from target depending on whether Cpk is less than or greater than Cp.
Now i go back to my assumption, and ask anyone to please confirm if –
k = ABSOLUTE VALUE of (Target – Process average)/[(USLLSL)/2] ?
0December 8, 2004 at 9:15 pm #112053Hi Sam,
The k formula does include the absolute value operator, meaning that k is always positive or zero. Thus, Cpk will always be less than, or equal to, Cp.
Cpk = Cp ( 1 – k )
Hope this helps.0December 9, 2004 at 12:17 am #112057
sam1tim315Member@sam1tim315 Include @sam1tim315 in your post and this person will
be notified via email.Hi Ross,
Thanks a lot for your confirmation. Now i understand the other definition/formula of Cpk (i.e. the lesser of USL – Mean/ ..or Mean – LSL/….).
sam0December 9, 2004 at 4:29 am #112075
Brian.P.M.Participant@Brian.P.M. Include @Brian.P.M. in your post and this person will
be notified via email.OK Lets try this again…
Cp=(USLLSL)/6*sigma hat
Cpk=min (USLx bar bar)/3*sigma hat OR (x bar barUSL)/3*sigma hat
Where sigma hat is the estimated standard deviation arrived at using the formula r bar/d2, d2 being a constant based on subgroup size.
The formulas given so far have used the actual sigma in the calculations which would give Pp & Ppk
The proccess must also be in statisical control in order for these indexes to be meaningful.
Because Cp & Cpk use estemates of the standard deviation based on the average range of the subgroups, proccess with small within variation but large between variaton will show a fair index. But if the same data are put in the Pp & Ppk formulas the index will drop like a stone.
If anyone is interested, I have a small data set that shows a Cp of ~1.7 but a Pp of ~.66
It also shows the importance of being in control as the data would show out of control points if charted.
My $0.02
Brian.P.M.
0August 11, 2006 at 2:45 am #141653
DesmondParticipant@Desmond Include @Desmond in your post and this person will
be notified via email.Cp is calcuated as (USL – LSL)/6*Std.Dev.
Cpl = (Mean – LSL)/3*Std.dev
Cpu=(USLMean)/3*Std.dev
Question:
What’s explanation behind the “6” used in Cp and the “3” used in Cpl/Cpu?
Appreciate someone can enlighten me on that.
Cheers and thx!
Rgrds,
Desmond.0August 11, 2006 at 2:54 am #141654Cp looks at the entire spread of the normal curve (both sides) which is why it uses 6x.
Cpk is the comparison of the two sides of the normal curve of which it takes the smaller (i.e Cpk = smaller of Cpu or Cpl). Cpu and Cpl are onesided which results in the 3x.0August 11, 2006 at 2:57 am #141655Difference between UCL and LCL = 6 standard deviation (total range)
Mean to LCL = 3 standard deviations, lower part of the range (Mean cuts the UCL and the LCL in half if distribution is normal)
Mean to UCL = 3 standard deviations, upper part of the range
I hope this helps.0August 11, 2006 at 4:25 am #141658
villageidiotMember@villageidiot Include @villageidiot in your post and this person will
be notified via email.In any 6S improvement you only do 2 things: either reduce the variation of process and/or center it. This allows you to move the distribution of your process within your specification limits (hopefully well within) and thus determines the degree to which it is capable of meeting your customers requirements.
All Cp and Cpk do is give you a basic comparison between what error is allowed by your customer (Tolerance) and what error is found in your current process (std deviation). That’s it. Cp tells you if it is even mathmatically possible for your process to be capable. Cpk tells you more realistically how capable your process is by taking into account how well it is centered around your target.
At Cp = 1.0 your process is theoretically possible of being fully capable, but not practically, as your process output will shift (long term variability, 1.5 shift, etc). Interpret the ratio accordingly. At Cpk of 1.33 (I believe), conventional wisdom tells you that your process is repeatable, in control, and thus, predictable. This is app. a 4 sigma level process and takes into account shifting of your process over time.
Verify this. It has been awhile since I have used it. Good luck.0August 11, 2006 at 5:12 am #141664
S. PraveenMember@S.Praveen Include @S.Praveen in your post and this person will
be notified via email.detais and graphs
0August 11, 2006 at 5:22 am #141667
villageidiotMember@villageidiot Include @villageidiot in your post and this person will
be notified via email.I have no idea what that means.
0August 11, 2006 at 3:36 pm #141679pictures speak louder
http://www.et.byu.edu/groups/mfg340/lessons/capability/8rulesforcpk.html0October 3, 2006 at 6:16 pm #144221I am using Cp and Cpk when testing production semiconductor chips. I use Cpk when running a reliability study running 1 part 100 times. Is Cpk still valid if I run 50 parts 1 time each? Is that a valid distribution? If not, what would be a correct quantity to run?
Thanks in advance,
Jeff0November 3, 2006 at 4:14 am #146456
vishal BhardwajMember@vishalBhardwaj Include @vishalBhardwaj in your post and this person will
be notified via email.Dear Sir,
Pls. send me the detail of Cp & Cpk.
I am very thanks full to you.
Thanks & Regards
Vishal Bhardwaj
0November 3, 2006 at 4:14 am #146457
vishal BhardwajMember@vishalBhardwaj Include @vishalBhardwaj in your post and this person will
be notified via email.Dear Sir,
Pls. send me the detail of Cp & Cpk.
I am very thanks full to you.
Thanks & Regards
Vishal Bhardwaj
0November 28, 2006 at 3:13 pm #148071
Jay ArthurParticipant@JayArthur Include @JayArthur in your post and this person will
be notified via email.Your post describes Cp and Cpk as calculated using stdev. However Pp and Ppk are calculated with stdev, but Cp and Cpk use sigma estimator the average of the ranges over the constant d2 (Rbar/d2). For correctness, your post should read:
Calculating Cpk and PpkCp = (USL – LSL)/6*SigmaEstCpl = (Mean – LSL)/3*SigmaEstCpu = (USLMean)/3*SigmaEstCpk = Min(Cpl,Cpu)
Pp = (USL – LSL)/6*Std.DevPpl = (Mean – LSL)/3*Std.devPpu = (USLMean)/3*Std.devPpk = Min(Cpl,Cpu)
AIAG standard formulas are available at: http://www.qimacros.com/formulas.html#histogram0November 28, 2006 at 3:15 pm #148073Jay – no self promoting. Thank you.
0November 28, 2006 at 3:26 pm #148075When calculating Pp and Ppk for a process capability study, overall standard deviation is typically adjusted by the bias correction factor c4, so there will be a slight difference in the overall standard deviation calculation for the smaller sample sizes.
0November 28, 2006 at 5:15 pm #148084
The ForceMember@TheForce Include @TheForce in your post and this person will
be notified via email.Cp – can it fit the spec
Cpk – does it fit the spec0November 28, 2006 at 5:57 pm #148087Before getting wrapped around the axle on this one, consider why you are using it….I am not a big fan of this particular metric…lots of reasons…talk to someone smarter than me to determine if you even want to get into this metric….can be misleading, confusing, and just plain wrong….good luck.
0November 28, 2006 at 9:28 pm #148101Cpk < 1.0 … can still be 100% in spec !
These are very misleading numbers.
The process MUST be stable for a process to be capable.0December 18, 2006 at 7:31 am #149143
lokesh madhokParticipant@lokeshmadhok Include @lokeshmadhok in your post and this person will
be notified via email.Dear Sir,
hello,
how are you ?
sir first pls explain me cp/cpk value,how to use.
regards!
lokesh
mobile no:98115111030March 22, 2007 at 6:47 pm #153771It is written as:
Cpl = (Mean – LSL)/3*Std.dev
Cpu=(USLMean)/3*Std.dev
But shouldn’t it be
Cpl = (Mean – LSL)/(3*Std.dev)
Cpu=(USLMean)/(3*Std.dev)
Notice the final parenthesis.0March 23, 2007 at 7:43 am #153800
Sunil AgrawalMember@SunilAgrawal Include @SunilAgrawal in your post and this person will
be notified via email.Cp gives how much process variation is fitting between specification limits. For example Cp=1 indicate that 99.73 % of process variation in fitting between specification limits.
While Cpk gives closeness of process mean to mean of specification limits in addition to information given by Cp.
For example your Cp=2 and your Cpk=1.5. It indicate that your 99.999998% of variation is fitting between specification limits but your process mean is away for specification mean 1.5 time of Std. deviation.0April 6, 2007 at 6:38 am #154485
Prashob JacobParticipant@PrashobJacob Include @PrashobJacob in your post and this person will
be notified via email.Sir
I would like to know how the formulas for Cmk and Cpk has ben derived? What is the basis if the formulas and who was the one who proposed the formulas. Ths is a part of my project.
Please reply at the earliest
With Regards
Prashob0June 8, 2007 at 11:19 am #157169Cp – can be simply defined as process capability mean how much a process can deliver good results.
Cpk – is process capability indiex.0July 14, 2007 at 5:46 am #158636
V ASHOK KUMARMember@VASHOKKUMAR Include @VASHOKKUMAR in your post and this person will
be notified via email.Please let me know what values should be considered for USL and LSL. My concern is whether the USL & LSL of the standard average weight should be considered or the actual average weight ‘s USL & LSL should be considered.
For Ex.
Standard WT: 700 mg. + / – 5%
USL : 735
LSL: 665
Actual Average Weight observed durng the run is 704 mg.
USL: 739.2
LSL: 668.8
PLease let me know which one is the right value
It would of great help if u cansolve my concern
V Ashok kUmar
Cipla
Patalganga
0July 14, 2007 at 11:18 am #158637
Bhupendra NemaParticipant@BhupendraNema Include @BhupendraNema in your post and this person will
be notified via email.Dear Mr. Ashok,
By Cp and Cpk we would always try to measure the process capabilities with respect to our specification(standard).
Thats why, always we should consider the standard specifications for the USL and LSL in Cp and Cpk calculation.
In your case standard spec is 700+5% and actual mean is 704 mg, So for Cp and Cpk calculation,
Average = 704 mg(Actual)
USL = 735 mg(standard)
LSL = 665 mg (standard)
Formule for calculation:
Cp=(USLLSL)/(6*Standard Deviation of mesured values)
Cpk=(USLAvg)/(3*Standard Deviation of mesured values) or
(AvgLSL)/(3*Standard Deviation of mesured values) whatever is lower.
Regards,
Bhupendra Nema
Crompton Greaves Ltd.
Bhopal (India)0 
AuthorPosts
The forum ‘General’ is closed to new topics and replies.