# Calculating Upper-CL/Lower-CL and Process Capability

D

#### dschneider-gamra

I am trying to calculate UCL/LCL and process capability and I dont know what data is "correct" to use.

I generate an SPC data charts that are each comprised of thousands of data points. The SPC chart presents range, cp, cpk, max, min, min, std dev.

If I am trying to determine the overall process capability and calculate an UCL/LCL should I use all l of the data points (numbering in the millions of data points) and generate the the statistical data from all of that data. (this would be very labor intensive to get all the data into one spreadsheet).

Or

Do I use the max/min range from the SPC data?

or

Do I use the Mean +/- 3 std dev

or do I perform a majic spell on the data....

Those are all the ideas that I have - hopefully I have supplied enough information on the data that I have and the question. If not please let me know.

Thanks for the help.

Dave

#### Marc

##### Hunkered Down for the Duration
Staff member
A quick "Bump". My Thanks in advance to any of you can help with this one.

A

I generate an SPC data charts that are each comprised of thousands of data points. The SPC chart presents range, cp, cpk, max, min, min, std dev.

Can you tell us what software is used for the 'SPC' chart? With everything else it reports, it seems like there might also be control limits somewhere. If not, then you'll want to look at the data sampling structure to determine the most correct way to determine the control limits.

Cheers,

#### Jim Wynne

Staff member
I am trying to calculate UCL/LCL and process capability and I dont know what data is "correct" to use.

I generate an SPC data charts that are each comprised of thousands of data points. The SPC chart presents range, cp, cpk, max, min, min, std dev.

If I am trying to determine the overall process capability and calculate an UCL/LCL should I use all l of the data points (numbering in the millions of data points) and generate the the statistical data from all of that data. (this would be very labor intensive to get all the data into one spreadsheet).

Or

Do I use the max/min range from the SPC data?

or

Do I use the Mean +/- 3 std dev

or do I perform a majic spell on the data....

Those are all the ideas that I have - hopefully I have supplied enough information on the data that I have and the question. If not please let me know.

Thanks for the help.

Dave
I'm not sure where to begin but, if you can answer some questions you'll get better help:
1. What's the manufacturing process?
2. How long does it take to accumulate millions of data points?
3. Are you sure you're dealing with a normal distribution?
4. You say you've generated an SPC chart. Have you used rational subgrouping or individuals? In either case, were samples taken at uniform intervals and charted in chronological order?
That's enough for now, I think.

#### bobdoering

Trusted Information Resource
I'm not sure where to begin but, if you can answer some questions you'll get better help:
1. What's the manufacturing process?
2. Are you sure you're dealing with a normal distribution?
3. You say you've generated an SPC chart. Have you used rational subgrouping or individuals? In either case, were samples taken at uniform intervals and charted in chronological order?
4. Can you supply some sample data?
This should be a boilerplate for all SPC questions! #### Darius

##### Quite Involved in Discussions do I perform a majic spell on the data....
I like that Normally, I would try the data it self, altho you mention Ranges and Min/Max as I can understand the data is charted as IXmR.

As Donald Wheeler said (more or less), there are not right and wrong Control Limits, the control limits are a reflexion of the process natural variation .

You can take "n" data points (let's say 120 data points) and obtain the statistics, and take another set of the data and compare the results, if the process looks more or less the same, you can take them as a good estimate.

The experts say that you shouldn't recalculate the control limit's unless there is a change on the behavior of the data (aka change of the mean, trends etc).

#### bobdoering

Trusted Information Resource
I also recommend looking at this, realizing that collecting the data is in the middle of the SPC implementation process, not the beginning.

D

#### dschneider-gamra

1) the mfg process is profile extrusion of plastic.
2) a data point is taken every 6" and we run 60'/min - so we get 120 data points each minute.
3) Each data point is ploted and the data is generated - max/min/cp/cpk etc.
4) not sure how to input data - I tried to insert but the post was rejected I know I get a CP/CPK for each run - how do I determine overall capability? How to I set an overall UCL/LCL based on all the data points - or do I use some of the calculated values.

The customer is saying we should have control limits +/-0.010" - I am trying to show them statistically that the control limits need to be calculated based on the data generated not a number pulled out of a hat.

By the way I should mention that my personal cabability (pc not cp) on understanding statistics = 0.002 #### bobdoering

Trusted Information Resource
I know I get a CP/CPK for each run - how do I determine overall capability? How to I set an overall UCL/LCL based on all the data points - or do I use some of the calculated values.
If you can use all of the data (not limited by the number of data points), then I would use them all.

The customer is saying we should have control limits +/-0.010" - I am trying to show them statistically that the control limits need to be calculated based on the data generated not a number pulled out of a hat.
Yeah, what is a customer thinking "specifying" control limits? That is an abject lack of SPC knowledge...unless they did some analysis of the data you gave them at some point in time.

D Calculating Lower and Upper 3 Sigma Control Limits Statistical Analysis Tools, Techniques and SPC 2
K Calculating UCL (Upper Control Limit) and LCL (Lower Control Limit) Statistical Analysis Tools, Techniques and SPC 14
D Calculating upper and lower confidence interval bounds Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
L Geometric Tolerancing and SPC - Calculating position upper and lower control limits Statistical Analysis Tools, Techniques and SPC 36
K Calculating Capability of a process when data is skewed in the upper range Capability, Accuracy and Stability - Processes, Machines, etc. 4
Calculating Heat Dissipation Manufacturing and Related Processes 1
Calculating Defect Rates ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 4
Formula for Calculating NoGo Major Diameter for UN gages Inspection, Prints (Drawings), Testing, Sampling and Related Topics 4
Calculating a weight for a machine for the CE label CE Marking (Conformité Européene) / CB Scheme 0
Remote Support - Calculating Number of Employees IATF 16949 - Automotive Quality Systems Standard 2
Calculating Tolerance of a Panel Meter with a 2 degrees of an Arc General Measurement Device and Calibration Topics 0
Calculating LCL, UCL, Cp, and Cpk in an Excel Spreadsheet Statistical Analysis Tools, Techniques and SPC 3
Product Development - When to start calculating Process Capability Capability, Accuracy and Stability - Processes, Machines, etc. 3
Calculating Reliability for Subsystems in Series Reliability Analysis - Predictions, Testing and Standards 15
Calculating (3rd Party) Audit Days for Company with Seasonal Employees General Auditing Discussions 3
T Formulas for Calculating Coefficients for RTD / PRTD Calibration According to ITS-90 General Measurement Device and Calibration Topics 5
J Calculating part variation from historic data for GRR study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Ta, Tb & R Square in Linearity Study Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 1
P Purpose of calculating Uncertainty value in calibration study Measurement Uncertainty (MU) 5
Calculating MTBF for plug-in PCI cards Reliability Analysis - Predictions, Testing and Standards 2
J Calculating impact force, expressed in lbs, of a 5 lb weight dropped 48 inches Inspection, Prints (Drawings), Testing, Sampling and Related Topics 1
M Calculating Cpk when sample size equals to 1 Capability, Accuracy and Stability - Processes, Machines, etc. 12
Calculating Cpk on Non-Normal Data Distribution Capability, Accuracy and Stability - Processes, Machines, etc. 10
A Calculating Accuracy for a Multimeter General Measurement Device and Calibration Topics 1
Calculating Coefficients for an RTD (Resistance Temperature Detector) Probe General Measurement Device and Calibration Topics 3
T Calculating Plating/Coating Weight for IMDS RoHS, REACH, ELV, IMDS and Restricted Substances 3
M Calculating Adequate Receiving Inspection Sample Size Statistical Analysis Tools, Techniques and SPC 2
M Calculating Capability of Delivery Performance Capability, Accuracy and Stability - Processes, Machines, etc. 5
K Advice on Calculating Control Chart Control Limits Statistical Analysis Tools, Techniques and SPC 13
B Calculating Combined DPMO and Sigma Level for Two or More Different Work Areas Six Sigma 3
G Calculating the Fraction Defective Confidence Interval for a Lot Statistical Analysis Tools, Techniques and SPC 2
A Calculating Combined Measurement Uncertainty - VDA 5 Measurement Uncertainty (MU) 1
B Calculating Precision to Tolerance Ratio Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 9
D DPPM - Calculating the Defect Rate of a Software Test Statistical Analysis Tools, Techniques and SPC 9
L FMEA - Calculating RPN = S*O*D FMEA and Control Plans 3
I Shewhart Constants vs Central Limit Theorem in calculating Control Limits Statistical Analysis Tools, Techniques and SPC 18
B Please share a template for calculating Cp Cpk Document Control Systems, Procedures, Forms and Templates 3
S Is a Stable Process (within Control Limits) required for Calculating Pp, Ppk? Capability, Accuracy and Stability - Processes, Machines, etc. 6
Calculating the IATF TS 16949 Recertification Audit Man Days - Clarification ISO 9000, ISO 9001, and ISO 9004 Quality Management Systems Standards 6
IEC60825 - Laser Safety - Calculating Maximum Permissible Exposure Correctly Other ISO and International Standards and European Regulations 1
P Calculating Process Capability from Data of attached spreadsheet Statistical Analysis Tools, Techniques and SPC 4
I AQL Formula in Excel - Calculating Sample Size based on an AQL Chart AQL - Acceptable Quality Level 8
D Formula for Calculating Warp Measurement of Corrugated Boxes General Measurement Device and Calibration Topics 1
Need Help Calculating and Implementing Cs - Wright's Index Statistical Analysis Tools, Techniques and SPC 11
Calculating an Uncertainty Budget for an Optical Comparator General Measurement Device and Calibration Topics 4
D One-Two-Three Sigma Control Limits - Calculating Control Limits for X-bar Charts Statistical Analysis Tools, Techniques and SPC 3
K Calculating (Approximate) Parallelism Inspection, Prints (Drawings), Testing, Sampling and Related Topics 10
S Calculating MTBF (Mean Time Between Failures) for the whole Production Unit Reliability Analysis - Predictions, Testing and Standards 2
J Need help calculating actual Ppk - Result of % GR&R=11.8% when conducting MSA Gage R&R (GR&R) and MSA (Measurement Systems Analysis) 11
M Calculating Cp and Cpk - Is my calculation correct? Capability, Accuracy and Stability - Processes, Machines, etc. 7