Cp and Cpk - Cpk is an index that should be applied to a stable process

D

Don Winton

#1
<font color=#0011dd>…except one X bar value which was below LCL, all other points are within Control limits </font>

<font color=#0011dd>Could some body explain how is it possible to have a good Cp and Cpk values in this condition.</font>

From Cpk.pdf in the FTP portion of this site:

Calculation of predictable process capability indices is dependent on the statistical control of the process. If the process is not in statistical control, then the results of the study are subject to fluctuate unpredictability.

Regards,

Don
------------------
I was better but I got over it.
 
Elsmar Forum Sponsor
J

john allen

#2
Cp and Cpks

When I did a X bar R chart with subgroup size of 5 and total lot size 50, except one X bar value which was below LCL, all other points are within Control limits. Interestingly, when I calculated Cp was 1.5 and Cpk was 1.38. Also, 7 individual X values are out of UCL and LCL limits.

Could some body explain how is it possible to have a good Cp and Cpk values in this condition.

Regards....
 
K

KenK - 2009

#3
Regarding "7 individual X values are out of UCL and LCL limits"

That is not so unusual since the raw X values follow an entirely different probability distribution (with larger variation) than that of the means. The control limits are based upon that distribution of the means, not of the individual X values.

With a sample size of 5, 18% of the individual data values would be expected to fall outside of +/-3 standard deviations OF THE MEAN. Of your 50 data points, 18% is 9, so you actually did better than expected.

For sample sizes of 10, 15, & 20, the respective percentages that might exceed the control limits are 34%, 44%, and 50%.

Ken K.
 
Q

QEgirl

#4
The Cp and Cpk are independent of the control chart....they are measures of the ability of the process distribution to stay within the product specifications, and they are not related to the control limits. Therefore, it is perfectly possible to have a "good" Cp and Cpk and yet have an out of control process. The only problem is, as mentioned before, that you can't really predict what your process is going to do unless it is in statistical control (i.e. within control limits).
 
A

AJLenarz

#5
Originally posted by QEgirl:
The Cp and Cpk are independent of the control chart....they are measures of the ability of the process distribution to stay within the product specifications, and they are not related to the control limits. Therefore, it is perfectly possible to have a "good" Cp and Cpk and yet have an out of control process.
I believe a CPK is an indicator for a stable process and should not be used to report on processes that are out of control. Be carefull on the use and application of "CPK"
 
A

Al Dyer

#6
If you have the time and the software a good experiment is to input all of your data into your program and and print our the Cp/Cpk values using different sample sizes, 2,3,4,5,6 etc... and review the data. This could give you a good mental picture of the relationship between Cp/Cpk and process control.

ASD...
 
A
#7
Measurement of Cp or Cpk, without the process being under Statistical Control is useless. One point on the X bar char bellow LCL indicates a presence of assignable cause of variation in the process. This cause needs to be identified and prevented from recurring. Recalculate the control limits omiting this data for which assignable cause has been identified. Possibly some other point may lie outside the new control limits. Repeat the process till no point lies ouside the control limits. Then the process would be under statistical control. Cp and Cpk can be evaluated under such a condition.

In case of bilateral tolerances, a different values of Cp and Cpk indicate the process average is not centered but near to either of the specification limits, there by resulting in either oversize or undersize products.
 
Q

QEgirl

#8
Agreed, Ajay. The Cp and Cpk are dependent upon a normal distribution. There are some conversion factors which can be used to approximate capability indices if you have an inherently skewed (e.g. stamping - tool wear) or kurtic process distribution.
 
Q

QEgirl

#10
John Allen-

Pearson distribution curves can be used to calculate capability for just about any shape of distribution. As long as you can determine the skewness and kurtosis of your process distribution, you can calculate a capability index by consulting a table containing the "standardized tails of Pearson curves". I've got a worksheet for calculating them, along with copies of the tables, copied from an old Quality Progress article. You might check the ASQ website and see if they have any info.
 
Thread starter Similar threads Forum Replies Date
I As a part of validation, should i include the acceptance criteria of CPk index >1.33? Qualification and Validation (including 21 CFR Part 11) 4
J Heat treatment inspection and Process capability index Cpk Inspection, Prints (Drawings), Testing, Sampling and Related Topics 2
C Composite Process Capability Index - Cpk is not additive Capability, Accuracy and Stability - Processes, Machines, etc. 3
V Quality Control Index - We thought that Cpk or Ppk would be good choices Capability, Accuracy and Stability - Processes, Machines, etc. 6
C Is Cpm (Taguchi's Index) Superior to Cpk and Ppk? Capability, Accuracy and Stability - Processes, Machines, etc. 1
L Using Cpk index for Accept / Reject Decisions Capability, Accuracy and Stability - Processes, Machines, etc. 1
C Capability index for one-sided specification? Cpk(max) = (USL-Xbar)/3*s ?? Capability, Accuracy and Stability - Processes, Machines, etc. 13
T SPC and GD&T - Machined parts - Is Cpk is the correct index? Capability, Accuracy and Stability - Processes, Machines, etc. 30
Q Capability - CPk comparison values Capability, Accuracy and Stability - Processes, Machines, etc. 12
D Tolerance definition based on expected Cp/cpk Reliability Analysis - Predictions, Testing and Standards 14
K Analysis variation among the lots during cpk calculation. Design and Development of Products and Processes 1
S Recommendation for user friendly Gaga R&R and Cpk software Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 10
A How to set up Continuous CpK monitoring of an injection mold process Reliability Analysis - Predictions, Testing and Standards 7
A Interpretation with regards to Ppk > Cpk Capability, Accuracy and Stability - Processes, Machines, etc. 14
U Cpk Calculation - I analyse a double seam cans Manufacturing and Related Processes 15
Proud Liberal Cp / Cpk on position using multiple MMC bonuses Capability, Accuracy and Stability - Processes, Machines, etc. 2
I Can we use pin gauges to measure an accurate Cpk ? Capability, Accuracy and Stability - Processes, Machines, etc. 9
T Final process capability results - What I am supposed to present? Cp and CpK? APQP and PPAP 11
F CpK on a manufactured assembly? Reliability Analysis - Predictions, Testing and Standards 7
M How is the Cmk and Cpk calculated? Statistical Analysis Tools, Techniques and SPC 0
O Cpk for Unilateral Tolerance Capability, Accuracy and Stability - Processes, Machines, etc. 9
B How to calculate CPK with n = 1 Manufacturing and Related Processes 2
O CPK with a P value less than 0.005 Manufacturing and Related Processes 8
A Short Term vs. Long Term SPC Study - Where is Cp and Cpk Defined Statistical Analysis Tools, Techniques and SPC 8
A Cpk Formula seems off, need help!? Capability, Accuracy and Stability - Processes, Machines, etc. 11
D Cp/Cpk on Gages for MSA (vs. Cg/Cgk) Capability, Accuracy and Stability - Processes, Machines, etc. 21
E Questions regarding Cpk Calculation - Should we be using LSL/USL or LCL/UCL? Capability, Accuracy and Stability - Processes, Machines, etc. 5
B Maximum Material Condition with Bonus - Determine Cpk for this one kpc Measurement Uncertainty (MU) 2
Hami812 Cpk Setting Tolerance - Cart before the Horse? (Wifi Routers) Capability, Accuracy and Stability - Processes, Machines, etc. 3
T Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
Proud Liberal Cpk on Position w/ MMC on Location and Size Capability, Accuracy and Stability - Processes, Machines, etc. 3
E SPC Production - Getting Cpk and Ppk Statistical Analysis Tools, Techniques and SPC 8
S Track Cpk trend Statistical Analysis Tools, Techniques and SPC 5
M Cp & Cpk about a diameter measured with min.,max. and average values. Capability, Accuracy and Stability - Processes, Machines, etc. 13
P Non-normal Data Cpk Statistical Analysis Tools, Techniques and SPC 5
alonFAI Cpk for Solder Paste Height Statistical Analysis Tools, Techniques and SPC 5
T Cpk for 0 as Ideal Value Statistical Analysis Tools, Techniques and SPC 1
P Profile Tolerance and CPK Manufacturing and Related Processes 3
V When to use Cp Cpk and Pp Ppk Capability, Accuracy and Stability - Processes, Machines, etc. 4
M Calculating Cpk when sample size equals to 1 Capability, Accuracy and Stability - Processes, Machines, etc. 12
S CpK in Minitab when 0.0 is the lower spec limit Statistical Analysis Tools, Techniques and SPC 7
I Best way to Chart an On-Going CPK Requirement Capability, Accuracy and Stability - Processes, Machines, etc. 4
U Cpk for Contaminant Mass - Washing Parts Capability, Accuracy and Stability - Processes, Machines, etc. 6
G Help with Cpk procedure Capability, Accuracy and Stability - Processes, Machines, etc. 3
J Calculation of σ within subgroups for Cpk in Minitab Using Minitab Software 1
Sean Kelley Value of Ppk or Cpk when targeting thinner material Statistical Analysis Tools, Techniques and SPC 4
B How to validate Microsoft Excel Spreadsheet Calculations for the Cpk value Capability, Accuracy and Stability - Processes, Machines, etc. 9
D Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10
D Cpk relation to Reliability Capability, Accuracy and Stability - Processes, Machines, etc. 4
B Process Capability - Changing Limits to Improve Cpk or find Root Causes? Capability, Accuracy and Stability - Processes, Machines, etc. 6

Similar threads

Top Bottom